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;;; advice.el --- advice mechanism for Emacs Lisp functions

;; Copyright (C) 1993 Free Software Foundation, Inc.

;; Author: Hans Chalupsky <hans@cs.buffalo.edu>
;; Created: 12 Dec 1992
;; Version: advice.el,v 2.1 1993/05/26 00:07:58 hans Exp
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;; Keywords: extensions, lisp, tools
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;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 2, or (at your option)
;; any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to
;; the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

;; LCD Archive Entry:
;; advice|Hans Chalupsky|hans@cs.buffalo.edu|
;; Advice mechanism for Emacs Lisp functions|
;; 1993/05/26 00:07:58|2.1|~/packages/advice.el.Z|


;;; Commentary:

;; @ Introduction:
;; ===============
;; This package implements a full-fledged Lisp-style advice mechanism
;; for Emacs Lisp. Advice is a clean and efficient way to modify the 
;; behavior of Emacs Lisp functions without having to keep  personal
;; modified copies of such functions around. A great number of such 
;; modifications can be achieved by treating the original function as a 
;; black box and specifying a different execution environment for it 
;; with a piece of advice. Think of a piece of advice as a kind of fancy
;; hook that you can attach to any function/macro/subr.

;; @ Highlights:
;; =============
;; - Clean definition of multiple, named before/around/after advices
;;   for functions, macros, subrs and special forms
;; - Full control over the arguments an advised function will receive,
;;   the binding environment in which it will be executed, as well as the
;;   value it will return.
;; - Allows re/definition of interactive behavior for functions and subrs
;; - Every piece of advice can have its documentation string which will be 
;;   combined with the original documentation of the advised function at
;;   call-time of `documentation' for proper command-key substitution.
;; - The execution of every piece of advice can be protected against error
;;   and non-local exits in preceding code or advices.
;; - Simple argument access either by name, or, more portable but as
;;   efficient, via access macros
;; - Allows the specification of a different argument list for the advised
;;   version of a function.
;; - Advised functions can be byte-compiled either at file-compile time
;;   (see preactivation) or activation time.
;; - Separation of advice definition and activation
;; - Provides generally accessible function definition (after) hooks
;; - Forward advice is possible (an application of definition hooks), that is
;;   as yet undefined or autoload functions can be advised without having to
;;   preload the file in which they are defined. 
;; - Forward redefinition is possible because around advice can be used to
;;   completely redefine a function.
;; - A caching mechanism for advised definition provides for cheap deactivation
;;   and reactivation of advised functions.
;; - Preactivation allows efficient construction and compilation of advised
;;   definitions at file compile time without giving up the flexibility of
;;   the advice mechanism.
;; - En/disablement mechanism allows the use of  different "views" of advised
;;   functions depending on what pieces of advice are currently en/disabled
;; - Provides manipulation mechanisms for sets of advised functions via 
;;   regular expressions that match advice names
;; - Allows definition of load-hooks for arbitrary Emacs Lisp files without
;;   modification of these files

;; @ How to get the latest advice.el:
;; ==================================
;; You can get the latest version of this package either via anonymous ftp
;; from ftp.cs.buffalo.edu (128.205.32.9) with pathname /pub/Emacs/advice.el,
;; or send email to hans@cs.buffalo.edu and I'll mail it to you.

;; @ Overview, or how to read this file:
;; =====================================
;; Advice has enough features now to justify an info file, however, I
;; didn't have the time yet to do all the necessary formatting. So,
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;; until I do have the time or some kind soul does it for me I crammed
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;; everything into the source file. Because about 50% of this file is
;; documentation it should be in outline-mode by default, but it is not.
;; If you choose to use outline-mode set `outline-regexp' to `";; @+"'
;; and use `M-x hide-body' to see just the headings. Use the various
;; other outline-mode functions to move around in the text. If you use
;; Lucid Emacs, you'll just have to wait until `selective-display'
;; works properly in order to be able to use outline-mode, sorry.
;;
;; And yes, I know: Documentation is for wimps.
;;
;; The four major sections of this file are:
;;
;;   @ This initial information       ...installation, customization etc.
;;   @ Advice documentation:          ...general documentation
;;   @ Foo games: An advice tutorial  ...teaches about advice by example
;;   @ Advice implementation:         ...actual code, yeah!!
;;
;; The latter three are actual headings which you can search for
;; directly in case outline-mode doesn't work for you.

;; @ Restrictions:
;; ===============
;; - Advised functions/macros/subrs will only exhibit their advised behavior
;;   when they are invoked via their function cell. This means that advice will
;;   not work for the following:
;;   + advised subrs that are called directly from other subrs or C-code 
;;   + advised subrs that got replaced with their byte-code during 
;;     byte-compilation (e.g., car)
;;   + advised macros which were expanded during byte-compilation before
;;     their advice was activated.
;; - This package was developed under GNU Emacs 18.59 and Lucid Emacs 19.6.
;;   It was adapted and tested for GNU Emacs 19.8 and seems to work ok for
;;   Epoch 4.2. For different Emacs environments your mileage may vary.

;; @ Credits:
;; ==========
;; This package is an extension and generalization of packages such as
;; insert-hooks.el written by Noah S. Friedman, and advise.el written by
;; Raul J. Acevedo. Some ideas used in here come from these packages,
;; others come from the various Lisp advice mechanisms I've come across
;; so far, and a few are simply mine.

;; @ Comments, suggestions, bug reports:
;; =====================================
;; If you find any bugs, have suggestions for new advice features, find the
;; documentation wrong, confusing, incomplete, or otherwise unsatisfactory,
;; have any questions about advice.el, or have otherwise enlightening
;; comments feel free to send me email at <hans@cs.buffalo.edu>.

;; @ Safety Rules and Emergency Exits:
;; ===================================
;; Before we begin: CAUTION!!
;; advice.el provides you with a lot of rope to hang yourself on very
;; easily accessible trees, so, here are a few important things you
;; should know: Once advice has been started with `ad-start-advice' it
;; generates advised definitions of the `documentation' function, and,
;; if definition hooks are enabled (e.g., for forward advice), also of
;; `defun', `defmacro' and `fset' (if you use Jamie Zawinski's (jwz)
;; optimizing byte-compiler as standardly used in GNU Emacs-19 and
;; Lucid Emacs-19 (Lemacs), then enabling definition hooks will also
;; redefine the `byte-code' subr). All these changes can be undone at
;; any time with `M-x ad-stop-advice'.
;; 
;; If you experience any strange behavior/errors etc. that you attribute to
;; advice.el or to some ill-advised function do one of the following:

;; - M-x ad-deactivate FUNCTION (if you have a definite suspicion what
;;                               function gives you problems)
;; - M-x ad-deactivate-all      (if you don't have a clue what's going wrong)
;; - M-x ad-stop-advice         (if you think the problem is related to the
;;                               advised functions used by advice.el itself)
;; - M-x ad-recover-normality   (for real emergencies)
;; - If none of the above solves your advice related problem go to another
;;   terminal, kill your Emacs process and send me some hate mail.

;; The first three measures have restarts, i.e., once you've figured out
;; the problem you can reactivate advised functions with either `ad-activate',
;; `ad-activate-all', or `ad-start-advice'. `ad-recover-normality' unadvises
;; everything so you won't be able to reactivate any advised functions, you'll
;; have to stick with their standard incarnations for the rest of the session.

;; IMPORTANT: With advice.el loaded always do `M-x ad-deactivate-all' before
;; you byte-compile a file, because advised special forms and macros can lead
;; to unwanted compilation results. When you are done compiling use
;; `M-x ad-activate-all' to go back to the advised state of all your 
;; advised functions.

;; RELAX: advice.el is pretty safe even if you are oblivious to the above.
;; I use it extensively and haven't run into any serious trouble in a long
;; time. Just wanted you to be warned.

;; @ Installation:
;; ===============
;; Put this file somewhere into your Emacs `load-path' and byte-compile it.
;; Both steps are mandatory! You cannot (and would not want to) run advice
;; uncompiled, and because there is bootstrapping going on the byte-compiler
;; needs to preload advice in order to compile it, hence, it has to find it
;; in your `load-path' (you can preload advice.el "by hand" before you compile
;; it if you don't want to put it into your `load-path'). Once you have
;; compiled advice put the following autoload declarations into your .emacs
;; to load it on demand
;;
;;    (autoload 'defadvice "advice" "Define a piece of advice" nil t)
;;    (autoload 'ad-add-advice "advice" "Add a piece of advice")
;;    (autoload 'ad-start-advice "advice" "Start advice magic" t)
;;
;; or explicitly load it with (require 'advice) or (load "advice").

;; @@ Preloading:
;; ==============
;; If you preload the complete advice.el or its autoloads into a dumped Emacs
;; image and you use jwz's byte-compiler make sure advice gets loaded after the
;; byte-compiler runtime support is loaded so that `ad-use-jwz-byte-compiler'
;; receives the proper initial value.

;; @ Customization:
;; ================
;; Part of the advice magic does not start until you call `ad-start-advice'
;; which you can either do interactively, explicitly in your .emacs, or by
;; putting
;;
;;    (setq ad-start-advice-on-load t)
;;
;; into your .emacs which will automatically start advice when the file gets
;; loaded.

;; If you want to be able to forward advise functions, that is to advise them
;; when they are not yet defined or defined as autoloads, then you should put 
;; the following into your .emacs
;;
;;    (setq ad-activate-on-definition t)
;;
;; which will activate all advice at the time the function gets actually 
;; defined/loaded. The value of this variable will not have any effect until
;; `ad-start-advice' gets executed.

;; If you use a v18 Emacs but use jwz's byte-compiler and want to use
;; forward advice make sure that `ad-use-jwz-byte-compiler' has a non-NIL
;; value after advice.el got loaded. If it doesn't set it explicitly in
;; your .emacs with
;;
;;     (setq ad-use-jwz-byte-compiler t)
;;
;; Also make sure that you read the paragraph on forward advice below to
;; find out about the trade-offs involved for this combination of features.

;; Look at the documentation of `ad-redefinition-action' for possible values
;; of this variable. Its default value is `warn' which will print a warning
;; message when an already defined advised function gets redefined with a
;; new original definition and de/activated.

;; @ Motivation:
;; =============
;; Before I go on explaining how advice works, here are four simple examples
;; how this package can be used. The first three are very useful, the last one
;; is just a joke:

;;(defadvice switch-to-buffer (before existing-buffers-only activate)
;;  "When called interactively switch to existing buffers only, unless 
;;when called with a prefix argument."
;;  (interactive 
;;   (list (read-buffer "Switch to buffer: " (other-buffer) 
;;                      (null current-prefix-arg)))))
;;
;;(defadvice switch-to-buffer (around confirm-non-existing-buffers activate)
;;  "Switch to non-existing buffers only upon confirmation."
;;  (interactive "BSwitch to buffer: ")
;;  (if (or (get-buffer (ad-get-arg 0))
;;          (y-or-n-p (format "`%s' does not exist, create? " (ad-get-arg 0))))
;;      ad-do-it))
;;
;;(defadvice find-file (before existing-files-only activate)
;;  "Find existing files only"
;;  (interactive "fFind file: "))
;;
;;(defadvice car (around interactive activate)
;;  "Make `car' an interactive function."
;;   (interactive "xCar of list: ")
;;   ad-do-it
;;   (if (interactive-p)
;;       (message "%s" ad-return-value)))


;; @ Advice documentation:
;; =======================
;; Below is general documentation of the various features of advice. For more
;; concrete examples check the corresponding sections in the tutorial part.

;; @@ Terminology:
;; ===============
;; - GNU Emacs-19: GNU's version of Emacs with major version 19
;; - Lemacs: Lucid's version of Emacs with major version 19
;; - v18: Any Emacs with major version 18 or built as an extension to that
;;        (such as Epoch)
;; - v19: Any Emacs with major version 19
;; - jwz: Jamie Zawinski - keeper of Lemacs and creator of the optimizing
;;        byte-compiler used in v19s.
;; - advices: Short for "pieces of advice".

;; @@ Defining a piece of advice with `defadvice':
;; ===============================================
;; The main means of defining a piece of advice is the macro `defadvice',
;; there is no interactive way of specifying a piece of advice.  A call to
;; `defadvice' has the following syntax which is similar to the syntax of
;; `defun/defmacro':
;;
;; (defadvice <function> (<class> <name> [<position>] [<arglist>] {<flags>}*)
;;   [ [<documentation-string>] [<interactive-form>] ]
;;   {<body-form>}* )

;; <function> is the name of the function/macro/subr to be advised.

;; <class> is the class of the advice which has to be one of `before',
;; `around', `after', `activation' or `deactivation' (the last two allow
;; definition of special act/deactivation hooks).

;; <name> is the name of the advice which has to be a non-NIL symbol.
;; Names uniquely identify a piece of advice in a certain advice class,
;; hence, advices can be redefined by defining an advice with the same class
;; and name. Advice names are global symbols, hence, the same name space
;; conventions used for function names should be applied.

;; An optional <position> specifies where in the current list of advices of
;; the specified <class> this new advice will be placed. <position> has to
;; be either `first', `last' or a number that specifies a zero-based
;; position (`first' is equivalent to 0). If no position is specified
;; `first' will be used as a default. If this call to `defadvice' redefines
;; an already existing advice (see above) then the position argument will
;; be ignored and the position of the already existing advice will be used.

;; An optional <arglist> which has to be a list can be used to define the
;; argument list of the advised function. This argument list should of
;; course be compatible with the argument list of the original function,
;; otherwise functions that call the advised function with the original
;; argument list in mind will break. If more than one advice specify an
;; argument list then the first one (the one with the smallest position)
;; found in the list of before/around/after advices will be used.

;; <flags> is a list of symbols that specify further information about the
;; advice. All flags can be specified with unambiguous initial substrings.
;;   `activate': Specifies that the advice information of the advised
;;              function should be activated right after this advice has been
;;              defined. In forward advices `activate' will be ignored. 
;;   `protect': Specifies that this advice should be protected against
;;              non-local exits and errors in preceding code/advices.
;;   `compile': Specifies that the advised function should be byte-compiled.
;;              This flag will be ignored unless `activate' is also specified.
;;   `disable': Specifies that the defined advice should be disabled, hence,
;;              it will not be used in an activation until somebody enables it.
;;   `preactivate': Specifies that the advised function should get preactivated
;;              at macro-expansion/compile time of this `defadvice'. This
;;              generates a compiled advised definition according to the
;;              current advice state which will be used during activation
;;              if appropriate. Only use this if the `defadvice' gets
;;              actually compiled (with a v18 byte-compiler put the `defadvice'
;;              into the body of a `defun' to accomplish proper compilation).

;; An optional <documentation-string> can be supplied to document the advice.
;; On call of the `documentation' function it will be combined with the
;; documentation strings of the original function and other advices.

;; An optional <interactive-form> form can be supplied to change/add
;; interactive behavior of the original function. If more than one advice
;; has an `(interactive ...)' specification then the first one (the one
;; with the smallest position) found in the list of before/around/after
;; advices will be used.

;; A possibly empty list of <body-forms> specifies the body of the advice in
;; an implicit progn. The body of an advice can access/change arguments,
;; the return value, the binding environment, and can have all sorts of 
;; other side effects.

;; @@ Assembling advised definitions:
;; ==================================
;; Suppose a function/macro/subr/special-form has N pieces of before advice,
;; M pieces of around advice and K pieces of after advice. Assuming none of
;; the advices is protected, its advised definition will look like this
;; (body-form indices correspond to the position of the respective advice in
;; that advice class):

;;    ([macro] lambda <arglist>
;;       [ [<advised-docstring>] [(interactive ...)] ]
;;       (let (ad-return-value)
;;         {<before-0-body-form>}*
;;               ....
;;         {<before-N-1-body-form>}*
;;         {<around-0-body-form>}*
;;            {<around-1-body-form>}*
;;                  ....
;;               {<around-M-1-body-form>}*
;;                  (setq ad-return-value
;;                        <apply original definition to <arglist>>)
;;               {<other-around-M-1-body-form>}*
;;                  ....
;;            {<other-around-1-body-form>}*
;;         {<other-around-0-body-form>}*
;;         {<after-0-body-form>}*
;;               ....
;;         {<after-K-1-body-form>}*
;;         ad-return-value))

;; Macros and special forms will be redefined as macros, hence the optional
;; [macro] in the beginning of the definition.

;; <arglist> is either the argument list of the original function or the
;; first argument list defined in the list of before/around/after advices.
;; The values of <arglist> variables can be accessed/changed in the body of
;; an advice by simply referring to them by their original name, however,
;; more portable argument access macros are also provided (see below).  For
;; subrs/special-forms for which neither explicit argument list definitions
;; are available, nor their documentation strings contain such definitions
;; (as they do v19s), `(&rest ad-subr-args)' will be used.

;; <advised-docstring> is an optional, special documentation string which will
;; be expanded into a proper documentation string upon call of `documentation'.

;; (interactive ...) is an optional interactive form either taken from the
;; original function or from a before/around/after advice. For advised
;; interactive subrs that do not have an interactive form specified in any
;; advice we have to use (interactive) and then call the subr interactively
;; if the advised function was called interactively, because the
;; interactive specification of subrs is not accessible. This is the only
;; case where changing the values of arguments will not have an affect
;; because they will be reset by the interactive specification of the subr.
;; If this is a problem one can always specify an interactive form in a
;; before/around/after advice to gain control over argument values that
;; were supplied interactively.
;; 
;; Then the body forms of the various advices in the various classes of advice
;; are assembled in order.  The forms of around advice L are normally part of
;; one of the forms of around advice L-1. An around advice can specify where
;; the forms of the wrapped or surrounded forms should go with the special
;; keyword `ad-do-it', which will be substituted with a `progn' containing the
;; forms of the surrounded code.

;; The innermost part of the around advice onion is 
;;      <apply original definition to <arglist>>
;; whose form depends on the type of the original function. The variable
;; `ad-return-value' will be set to its result. This variable is visible to
;; all pieces of advice which can access and modify it before it gets returned.
;; 
;; The semantic structure of advised functions that contain protected pieces
;; of advice is the same. The only difference is that `unwind-protect' forms
;; make sure that the protected advice gets executed even if some previous
;; piece of advice had an error or a non-local exit. If any around advice is
;; protected then the whole around advice onion will be protected.

;; @@ Argument access in advised functions:
;; ========================================
;; As already mentioned, the simplest way to access the arguments of an
;; advised function in the body of an advice is to refer to them by name. To
;; do that, the advice programmer needs to know either the names of the
;; argument variables of the original function, or the names used in the
;; argument list redefinition given in a piece of advice. While this simple
;; method might be sufficient in many cases, it has the disadvantage that it
;; is not very portable because it hardcodes the argument names into the
;; advice. If the definition of the original function changes the advice
;; might break even though the code might still be correct. Situations like
;; that arise, for example, if one advises a subr like `eval-region' which
;; gets redefined in a non-advice style into a function by the edebug
;; package. If the advice assumes `eval-region' to be a subr it might break
;; once edebug is loaded. Similar situations arise when one wants to use the
;; same piece of advice across different versions of Emacs. Some subrs in a
;; v18 Emacs are functions in v19 and vice versa, but for the most part the
;; semantics remain the same, hence, the same piece of advice might be usable
;; in both Emacs versions.

;; As a solution to that advice provides argument list access macros that get
;; translated into the proper access forms at activation time, i.e., when the
;; advised definition gets constructed. Access macros access actual arguments
;; by position regardless of how these actual argument get distributed onto
;; the argument variables of a function. The rational behind this is that in
;; Emacs Lisp the semantics of an argument is strictly determined by its
;; position (there are no keyword arguments).

;; Suppose the function `foo' is defined as
;;
;;    (defun foo (x y &optional z &rest r) ....)
;;
;; and is then called with
;;
;;    (foo 0 1 2 3 4 5 6)

;; which means that X=0, Y=1, Z=2 and R=(3 4 5 6). The assumption is that
;; the semantics of an actual argument is determined by its position. It is
;; this semantics that has to be known by the advice programmer. Then s/he
;; can access these arguments in a piece of advice with some of the
;; following macros (the arrows indicate what value they will return):

;;    (ad-get-arg 0) -> 0
;;    (ad-get-arg 1) -> 1
;;    (ad-get-arg 2) -> 2
;;    (ad-get-arg 3) -> 3
;;    (ad-get-args 2) -> (2 3 4 5 6)
;;    (ad-get-args 4) -> (4 5 6)

;; `(ad-get-arg <position>)' will return the actual argument that was supplied
;; at <position>, `(ad-get-args <position>)' will return the list of actual
;; arguments supplied starting at <position>. Note that these macros can be
;; used without any knowledge about the form of the actual argument list of
;; the original function.

;; Similarly, `(ad-set-arg <position> <value-form>)' can be used to set the
;; value of the actual argument at <position> to <value-form>. For example,
;;
;;   (ad-set-arg 5 "five")
;;
;; will have the effect that R=(3 4 "five" 6) once the original function is
;; called. `(ad-set-args <position> <value-list-form>)' can be used to set
;; the list of actual arguments starting at <position> to <value-list-form>.
;; For example,
;;
;;   (ad-set-args 0 '(5 4 3 2 1 0))
;;
;; will have the effect that X=5, Y=4, Z=3 and R=(2 1 0) once the original
;; function is called.

;; All these access macros are text macros rather than real Lisp macros. When
;; the advised definition gets constructed they get replaced with actual access
;; forms depending on the argument list of the advised function, i.e., after
;; that argument access is in most cases as efficient as using the argument
;; variable names directly.

;; @@@ Accessing argument bindings of arbitrary functions:
;; =======================================================
;; Some functions (such as `trace-function' defined in trace.el) need a
;; method of accessing the names and bindings of the arguments of an
;; arbitrary advised function. To do that within an advice one can use the
;; special keyword `ad-arg-bindings' which is a text macro that will be
;; substituted with a form that will evaluate to a list of binding
;; specifications, one for every argument variable.  These binding
;; specifications can then be examined in the body of the advice.  For
;; example, somewhere in an advice we could do this:
;;
;;   (let* ((bindings ad-arg-bindings)
;;          (firstarg (car bindings))
;;          (secondarg (car (cdr bindings))))
;;     ;; Print info about first argument
;;     (print (format "%s=%s (%s)"
;;                    (ad-arg-binding-field firstarg 'name)
;;                    (ad-arg-binding-field firstarg 'value)
;;                    (ad-arg-binding-field firstarg 'type)))
;;     ....)
;;
;; The `type' of an argument is either `required', `optional' or `rest'.
;; Wherever `ad-arg-bindings' appears a form will be inserted that evaluates
;; to the list of bindings, hence, in order to avoid multiple unnecessary
;; evaluations one should always bind it to some variable.

;; @@@ Argument list mapping:
;; ==========================
;; Because `defadvice' allows the specification of the argument list of the
;; advised function we need a mapping mechanism that maps this argument list
;; onto that of the original function. For example, somebody might specify
;; `(sym newdef)' as the argument list of `fset', while advice might use
;; `(&rest ad-subr-args)' as the argument list of the original function
;; (depending on what Emacs version is used). Hence SYM and NEWDEF have to
;; be properly mapped onto the &rest variable when the original definition is
;; called. Advice automatically takes care of that mapping, hence, the advice 
;; programmer can specify an argument list without having to know about the
;; exact structure of the original argument list as long as the new argument
;; list takes a compatible number/magnitude of actual arguments.

;; @@@ Definition of subr argument lists:
;; ======================================
;; When advice constructs the advised definition of a function it has to
;; know the argument list of the original function. For functions and macros
;; the argument list can be determined from the actual definition, however,
;; for subrs there is no such direct access available. In Lemacs and for some
;; subrs in GNU Emacs-19 the argument list of a subr can be determined from
;; its documentation string, in a v18 Emacs even that is not possible. If
;; advice cannot at all determine the argument list of a subr it uses
;; `(&rest ad-subr-args)' which will always work but is inefficient because
;; it conses up arguments. The macro `ad-define-subr-args' can be used by
;; the advice programmer to explicitly tell advice about the argument list
;; of a certain subr, for example,
;;
;;    (ad-define-subr-args 'fset '(sym newdef))
;;
;; is used by advice itself to tell a v18 Emacs about the arguments of `fset'.
;; The following can be used to undo such a definition:
;;
;;    (ad-undefine-subr-args 'fset)
;;
;; The argument list definition is stored on the property list of the subr
;; name symbol. When an argument list could be determined from the
;; documentation string it will be cached under that property. The general
;; mechanism for looking up the argument list of a subr is the following:
;; 1) look for a definition stored on the property list
;; 2) if that failed try to infer it from the documentation string and
;;    if successful cache it on the property list
;; 3) otherwise use `(&rest ad-subr-args)'

;; @@ Activation and deactivation:
;; ===============================
;; The definition of an advised function does not change until all its advice
;; gets actually activated. Activation can either happen with the `activate'
;; flag specified in the `defadvice', with an explicit call or interactive
;; invocation of `ad-activate', or if forward advice is enabled (i.e., the
;; value of `ad-activate-on-definition' is t) at the time an already advised
;; function gets defined.

;; When a function gets first activated its original definition gets saved,
;; all defined and enabled pieces of advice will get combined with the
;; original definition, the resulting definition might get compiled depending
;; on some conditions described below, and then the function will get
;; redefined with the advised definition.  This also means that undefined
;; functions cannot get activated even though they might be already advised.

;; The advised definition will get compiled either if `ad-activate' was called
;; interactively with a prefix argument, or called explicitly with its second
;; argument as t, or, if this was a case of forward advice if the original
;; definition of the function was compiled. If the advised definition was
;; constructed during "preactivation" (see below) then that definition will
;; be already compiled because it was constructed during byte-compilation of
;; the file that contained the `defadvice' with the `preactivate' flag.

;; `ad-deactivate' can be used to back-define an advised function to its
;; original definition. It can be called interactively or directly. Because
;; `ad-activate' caches the advised definition the function can be
;; reactivated via `ad-activate' with only minor overhead (it is checked
;; whether the current advice state is consistent with the cached
;; definition, see the section on caching below).

;; `ad-activate-regexp' and `ad-deactivate-regexp' can be used to de/activate
;; all currently advised function that have a piece of advice with a name that
;; contains a match for a regular expression. These functions can be used to
;; de/activate sets of functions depending on certain advice naming
;; conventions.

;; Finally, `ad-activate-all' and `ad-deactivate-all' can be used to
;; de/activate all currently advised functions. These are useful to
;; (temporarily) return to an un/advised state.

;; @@@ Reasons for the separation of advice definition and activation:
;; ===================================================================
;; As already mentioned, advising happens in two stages:

;;   1) definition of various pieces of advice
;;   2) activation of all advice currently defined and enabled

;; The advantage of this is that various pieces of advice can be defined
;; before they get combined into an advised definition which avoids
;; unnecessary constructions of intermediate advised definitions. The more
;; important advantage is that it allows the implementation of forward advice.
;; Advice information for a certain function accumulates as the value of the
;; `advice-info' property of the function symbol. This accumulation is
;; completely independent of the fact that that function might not yet be
;; defined. The special forms `defun' and `defmacro' have been advised to
;; check whether the function/macro they defined had advice information
;; associated with it. If so and forward advice is enabled, the original
;; definition will be saved, and then the advice will be activated. When a
;; file is loaded in a v18 Emacs the functions/macros it defines are also
;; defined with calls to `defun/defmacro'.  Hence, we can forward advise
;; functions/macros which will be defined later during a load/autoload of some
;; file (for compiled files generated by jwz's byte-compiler in a v19 Emacs
;; this is slightly more complicated but the basic idea is the same).

;; @@ Enabling/disabling pieces or sets of advice:
;; ===============================================
;; A major motivation for the development of this advice package was to bring
;; a little bit more structure into the function overloading chaos in Emacs
;; Lisp. Many packages achieve some of their functionality by adding a little
;; bit (or a lot) to the standard functionality of some Emacs Lisp function.
;; ange-ftp is a very popular package that achieves its magic by overloading
;; most Emacs Lisp functions that deal with files. A popular function that's
;; overloaded by many packages is `expand-file-name'. The situation that one
;; function is multiply overloaded can arise easily.

;; Once in a while it would be desirable to be able to disable some/all
;; overloads of a particular package while keeping all the rest.  Ideally -
;; at least in my opinion - these overloads would all be done with advice,
;; I know I am dreaming right now... In that ideal case the enable/disable
;; mechanism of advice could be used to achieve just that.

;; Every piece of advice is associated with an enablement flag. When the
;; advised definition of a particular function gets constructed (e.g., during
;; activation) only the currently enabled pieces of advice will be considered.
;; This mechanism allows one to have different "views" of an advised function
;; dependent on what pieces of advice are currently enabled.

;; Another motivation for this mechanism is that it allows one to define a
;; piece of advice for some function yet keep it dormant until a certain
;; condition is met. Until then activation of the function will not make use
;; of that piece of advice. Once the condition is met the advice can be
;; enabled and a reactivation of the function will add its functionality as
;; part of the new advised definition. For example, the advices of `defun'
;; etc. used by advice itself will stay disabled until `ad-start-advice' is
;; called and some variables have the proper values.  Hence, if somebody
;; else advised these functions too and activates them the advices defined
;; by advice will get used only if they are intended to be used.

;; The main interface to this mechanism are the interactive functions
;; `ad-enable-advice' and `ad-disable-advice'. For example, the following
;; would disable a particular advice of the function `foo':
;;
;;    (ad-disable-advice 'foo 'before 'my-advice)
;;
;; This call by itself only changes the flag, to get the proper effect in
;; the advised definition too one has to activate `foo' with
;;
;;    (ad-activate 'foo)
;;
;; or interactively. To disable whole sets of advices one can use a regular
;; expression mechanism. For example, let us assume that ange-ftp actually
;; used advice to overload all its functions, and that it used the
;; "ange-ftp-" prefix for all its advice names, then we could temporarily
;; disable all its advices with
;;
;;    (ad-disable-regexp "^ange-ftp-")
;;
;; and the following call would put that actually into effect:
;;
;;    (ad-activate-regexp "^ange-ftp-")
;;
;; A saver way would have been to use
;;
;;    (ad-update-regexp "^ange-ftp-")
;;
;; instead which would have only reactivated currently actively advised
;; functions, but not functions that were currently deactivated. All these
;; functions can also be called interactively.

;; A certain piece of advice is considered a match if its name contains a
;; match for the regular expression. To enable ange-ftp again we would use
;; `ad-enable-regexp' and then activate or update again.

;; @@ Forward advice, function definition hooks:
;; =============================================
;; Because most Emacs Lisp packages are loaded on demand via an autoload
;; mechanism it is essential to be able to "forward advise" functions.
;; Otherwise, proper advice definition and activation would make it necessary
;; to preload every file that defines a certain function before it can be
;; advised, which would partly defeat the purpose of the advice mechanism.

;; In the following, "forward advice" always implies its automatic activation
;; once a function gets defined, and not just the accumulation of advice
;; information for a possibly undefined function.

;; Advice implements forward advice mainly via the following: 1) Separation
;; of advice definition and activation that makes it possible to accumulate
;; advice information without having the original function already defined,
;; 2) special versions of the function defining functions `defun', `defmacro'
;; and `fset' that check for advice information whenever they define a
;; function. If advice information was found and forward advice is enabled
;; then the advice will immediately get activated when the function gets
;; defined.

;; @@@ Enabling forward advice:
;; ============================
;; Forward advice is enabled by setting `ad-activate-on-definition' to t
;; and then calling `ad-start-advice' which can either be done interactively,
;; directly with `(ad-start-advice)' in your .emacs, or by setting
;; `ad-start-advice-on-load' to t before advice gets loaded. For example,
;; putting the following into your .emacs will enable forward advice:
;;
;;    (setq ad-start-advice-on-load t)
;;    (setq ad-activate-on-definition t)
;;
;; "Activation on definition" means, that whenever a function gets defined
;; with either `defun', `defmacro', `fset' or by loading a byte-compiled
;; file, and the function has some advice-info stored with it then that
;; advice will get activated right away.

;; If jwz's byte-compiler is used then `ad-use-jwz-byte-compiler' should
;; be t in order to make forward advice work with functions defined in
;; compiled files generated by that compiler. In v19s which use this
;; compiler the value of this variable will be correct automatically.
;; If you use a v18 Emacs in conjunction with jwz's compiler and you want
;; to use forward advice then you should check its value after loading
;; advice. If it is nil set it explicitly with
;;
;;    (setq ad-use-jwz-byte-compiler t)
;;
;; along with `ad-activate-on-definition' before you start advice (see above).

;; IMPORTANT: A v18 Emacs + jwz's compiler + forward advice means performance
;;            tradeoffs which are described below.

;; @@@ Forward advice with compiled files generated by jwz's byte-compiler:
;; ========================================================================
;; The v18 byte-compiler only uses `defun/defmacro' to define compiled
;; functions, hence, providing advised versions of these functions was
;; sufficient to achieve forward advice. With the advent of Jamie Zawinski's
;; optimizing byte-compiler which is now standardly used in GNU Emacs-19 and
;; Lemacs things became more complicated. jwz's compiler defines functions
;; in hunks of byte-code without explicit usage of `defun/defmacro'. To
;; still provide forward advice even in this scenario, advice defines an
;; advised version of the `byte-code' subr that scans its arguments for
;; function definitions during the loading of compiled files. While this is
;; no problem in a v19 Emacs, because it uses a new datatype for compiled
;; code objects and the `byte-code' subr is only rarely used at all, it
;; presents a major problem in a v18 Emacs because there calls to
;; `byte-code' are the only means of executing compiled code (every body of
;; a compiled function contains a call to `byte-code'). Because the advised
;; `byte-code' has to perform some extra checks every call to a compiled
;; function becomes more expensive.

;; Enabling forward advice leads to performance degradation in the following
;; situations:
;; - A v18 Emacs is used and the value of `ad-use-jwz-byte-compiler' is t
;;   (either because jwz's byte-compiler is used instead of the standard v18
;;   compiler, or some compiled files generated by jwz's compiler are used).
;; - A v19 Emacs is used with some old-style v18 compiled files.
;; Some performance experiments I conducted showed that function call intensive
;; code (such as the highly recursive byte-compiler itself) slows down by a 
;; factor of 1.8. Function call intensive code that runs while a file gets
;; loaded can slow down by a factor of 6! For the v19 scenario this performance
;; lossage would only apply to code that was loaded from old v18 compiled
;; files.

;; MORAL: If you use a v18 Emacs in conjunction with jwz's byte-compiler you
;; should think twice whether you really need forward advice. There are some
;; alternatives to forward advice described below that might give you what
;; you need without the loss of performance (that performance loss probably
;; outweighs by far any performance gain due to the optimizing nature of jwz's
;; compiler).

;; @@@ Alternatives to automatic activation of forward advice:
;; ===========================================================
;; If you use a v18 Emacs in conjunction with jwz's compiler, or you simply
;; don't trust the automatic activation mechanism of forward advice, then
;; you can use some of the following alternatives to get around that:
;; - Preload the file that contains the definition of the function that you
;;   want to advice. Inelegant and wasteful, but it works.
;; - If the package that contains the definition of the function you want to
;;   advise has any mode hooks, and the advised function is only used once such
;;   a mode has been entered, then you can activate the advice in the mode 
;;   hook. Just put a form like `(ad-activate 'my-advised-fn t)' into the
;;   hook definition. The caching mechanism will reuse advised definitions,
;;   so calling that mode hook over and over again will not construct
;;   advised definitions over and over again, so you won't loose any
;;   performance.
;; - If your Emacs comes with file load hooks (such as v19's
;;   `after-load-alist' mechanism), then you can put the activation form
;;   into that, for example, add `("myfile" (ad-activate 'my-advised-fn t))'
;;   to it to activate the advice right ater "myfile" got loaded.

;; @@@ Function definition hooks:
;; ==============================
;; Automatic activation of forward advice is implemented as an application
;; of a more general function definition hook mechanism. After a function
;; gets re/defined with `defun/defmacro/fset' or via a hunk of byte-code
;; during the loading of a byte-compiled file, and function definition hooks
;; are enabled, then all hook functions stored in `ad-definition-hooks' are
;; run with the variable `ad-defined-function' bound to the name of the 
;; currently defined function.

;; Function definition hooks can be enabled with
;;
;;    (setq ad-enable-definition-hooks t)
;;
;; before advice gets started with `ad-start-advice'. Setting 
;; `ad-activate-on-definition' to t automatically enables definition hooks
;; regardless of the value of `ad-enable-definition-hooks'.

;; @@@ Wish list:
;; ==============
;; - The implementation of definition hooks for v19 compiled files would be
;;   safer if jwz's byte-compiler used something like `byte-code-tl' instead
;;   of `byte-code' to execute hunks of function defining byte-code at the
;;   top level of compiled files.
;; - Definition hooks should be implemented directly as part of the C-code
;;   that implements `fset', because then advice.el wouldn't have to use all
;;   these dirty hacks to achieve this functionality.

;; @@ Caching of advised definitions:
;; ==================================
;; After an advised definition got constructed it gets cached as part of the
;; advised function's advice-info so it can be reused, for example, after an
;; intermediate deactivation. Because the advice-info of a function might
;; change between the time of caching and reuse a cached definition gets
;; a cache-id associated with it so it can be verified whether the cached
;; definition is still valid (the main application of this is preactivation
;; - see below).

;; When an advised function gets activated and a verifiable cached definition
;; is available, then that definition will be used instead of creating a new
;; advised definition from scratch. If you want to make sure that a new
;; definition gets constructed then you should use `ad-clear-cache' before you
;; activate the advised function.

;; @@ Preactivation:
;; =================
;; Constructing an advised definition is moderately expensive. In a situation
;; where one package defines a lot of advised functions it might be
;; prohibitively expensive to do all the advised definition construction at
;; runtime. Preactivation is a mechanism that allows compile-time construction
;; of compiled advised definitions that can be activated cheaply during
;; runtime. Preactivation uses the caching mechanism to do that. Here's how it
;; works:

;; When the byte-compiler compiles a `defadvice' that has the `preactivate'
;; flag specified, it uses the current original definition of the advised
;; function plus the advice specified in this `defadvice' (even if it is
;; specified as disabled) and all other currently enabled pieces of advice to
;; construct an advised definition and an identifying cache-id and makes them
;; part of the `defadvice' expansion which will then be compiled by the
;; byte-compiler (to ensure that in a v18 emacs you have to put the
;; `defadvice' inside a `defun' to get it compiled and then you have to call
;; that compiled `defun' in order to actually execute the `defadvice'). When
;; the file with the compiled, preactivating `defadvice' gets loaded the
;; precompiled advised definition will be cached on the advised function's
;; advice-info. When it gets activated (can be immediately on execution of the
;; `defadvice' or any time later) the cache-id gets checked against the
;; current state of advice and if it is verified the precompiled definition
;; will be used directly (the verification is pretty cheap). If it couldn't get
;; verified a new advised definition for that function will be built from
;; scratch, hence, the efficiency added by the preactivation mechanism does
;; not at all impair the flexibility of the advice mechanism.

;; MORAL: In order get all the efficiency out of preactivation the advice
;;        state of an advised function at the time the file with the
;;        preactivating `defadvice' gets byte-compiled should be exactly
;;        the same as it will be when the advice of that function gets
;;        actually activated. If it is not there is a high chance that the
;;        cache-id will not match and hence a new advised definition will
;;        have to be constructed at runtime.

;; Preactivation and forward advice do not contradict each other. It is
;; perfectly ok to load a file with a preactivating `defadvice' before the
;; original definition of the advised function is available. The constructed
;; advised definition will be used once the original function gets defined and
;; its advice gets activated. The only constraint is that at the time the
;; file with the preactivating `defadvice' got compiled the original function
;; definition was available.

;; TIPS: Here are some indications that a preactivation did not work the way
;;       you intended it to work:
;;       - Activation of the advised function takes longer than usual/expected
;;       - The byte-compiler gets loaded while an advised function gets
;;         activated
;;       - `byte-compile' is part of the `features' variable even though you
;;         did not use the byte-compiler
;;       Right now advice does not provide an elegant way to find out whether
;;       and why a preactivation failed. What you can do is to trace the
;;       function `ad-cache-id-verification-code' (with the function
;;       `trace-function-background' defined in my trace.el package) before
;;       any of your advised functions get activated. After they got
;;       activated check whether all calls to `ad-cache-id-verification-code'
;;       returned `verified' as a result. Other values indicate why the
;;       verification failed which should give you enough information to
;;       fix your preactivation/compile/load/activation sequence.

;; IMPORTANT: There is one case (that I am aware of) that can make 
;; preactivation fail, i.e., a preconstructed advised definition that does
;; NOT match the current state of advice gets used nevertheless. That case
;; arises if one package defines a certain piece of advice which gets used
;; during preactivation, and another package incompatibly redefines that 
;; very advice (i.e., same function/class/name), and it is the second advice
;; that is available when the preconstructed definition gets activated, and
;; that was the only definition of that advice so far (`ad-add-advice' 
;; catches advice redefinitions and clears the cache in such a case). 
;; Catching that would make the cache verification too expensive.

;; MORAL-II: Redefining somebody else's advice is BAAAAD (to speak with
;; George Walker Bush), and why would you redefine your own advice anyway?
;; Advice is a mechanism to facilitate function redefinition, not advice
;; redefinition (wait until I write meta-advice.el :-). If you really have
;; to undo somebody else's advice try to write a "neutralizing" advice.

;; @@ Advising macros and special forms and other dangerous things:
;; ================================================================
;; Look at the corresponding tutorial sections for more information on
;; these topics. Here it suffices to point out that the special treatment
;; of macros and special forms by the byte-compiler can lead to problems
;; when they get advised. Macros can create problems because they get
;; expanded at compile time, hence, they might not have all the necessary
;; runtime support and such advice cannot be de/activated or changed as
;; it is possible for functions. Special forms create problems because they
;; have to be advised "into" macros, i.e., an advised special form is a
;; implemented as a macro, hence, in most cases the byte-compiler will
;; not recognize it as a special form anymore which can lead to very strange
;; results.
;;
;; MORAL: - Only advise macros or special forms when you are absolutely sure
;;          what you are doing.
;;        - As a safety measure, always do `ad-deactivate-all' before you
;;          byte-compile a file to make sure that even if some inconsiderate
;;          person advised some special forms you'll get proper compilation
;;          results. After compilation do `ad-activate-all' to get back to
;;          the previous state.

;; @@ Adding a piece of advice with `ad-add-advice':
;; =================================================
;; The non-interactive function `ad-add-advice' can be used to add a piece of
;; advice to some function without using `defadvice'. This is useful if advice
;; has to be added somewhere by a function (also look at `ad-make-advice').

;; @@ Activation/deactivation advices, file load hooks:
;; ====================================================
;; There are two special classes of advice called `activation' and
;; `deactivation'. The body forms of these advices are not included into the
;; advised definition of a function, rather they are assembled into a hook
;; form which will be evaluated whenever the advice-info of the advised
;; function gets activated or deactivated. One application of this mechanism
;; is to define file load hooks for files that do not provide such hooks
;; (v19s already come with a general file-load-hook mechanism, v18s don't).
;; For example, suppose you want to print a message whenever `file-x' gets
;; loaded, and suppose the last function defined in `file-x' is
;; `file-x-last-fn'.  Then we can define the following advice:
;;
;;   (defadvice file-x-last-fn (activation file-x-load-hook)
;;      "Executed whenever file-x is loaded"
;;      (if load-in-progress (message "Loaded file-x")))
;;
;; This will constitute a forward advice for function `file-x-last-fn' which
;; will get activated when `file-x' is loaded (only if forward advice is
;; enabled of course). Because there are no "real" pieces of advice
;; available for it, its definition will not be changed, but the activation
;; advice will be run during its activation which is equivalent to having a
;; file load hook for `file-x'.

;; @@ Summary of main advice concepts:
;; ===================================
;; - Definition:
;;     A piece of advice gets defined with `defadvice' and added to the
;;     `advice-info' property of a function.
;; - Enablement:
;;     Every piece of advice has an enablement flag associated with it. Only
;;     enabled advices are considered during construction of an advised
;;     definition.
;; - Activation:
;;     Redefine an advised function with its advised definition. Constructs
;;     an advised definition from scratch if no verifiable cached advised
;;     definition is available and caches it.
;; - Deactivation:
;;     Back-define an advised function to its original definition.
;; - Update:
;;     Reactivate an advised function but only if its advice is currently 
;;     active. This can be used to bring all currently advised function up
;;     to date with the current state of advice without also activating
;;     currently deactivated functions.
;; - Caching:
;;     Is the saving of an advised definition and an identifying cache-id so
;;     it can be reused, for example, for activation after deactivation.
;; - Preactivation:
;;     Is the construction of an advised definition according to the current
;;     state of advice during byte-compilation of a file with a preactivating
;;     `defadvice'. That advised definition can then rather cheaply be used
;;     during activation without having to construct an advised definition
;;     from scratch at runtime.

;; @@ Summary of interactive advice manipulation functions:
;; ========================================================
;; The following interactive functions can be used to manipulate the state
;; of advised functions (all of them support completion on function names,
;; advice classes and advice names):

;; - ad-activate to activate the advice of a FUNCTION
;; - ad-deactivate to deactivate the advice of a FUNCTION
;; - ad-update   to activate the advice of a FUNCTION unless it was not
;;               yet activated or is currently deactivated.
;; - ad-unadvise deactivates a FUNCTION and removes all of its advice 
;;               information, hence, it cannot be activated again
;; - ad-recover  tries to redefine a FUNCTION to its original definition and
;;               discards all advice information (a low-level `ad-unadvise').
;;               Use only in emergencies.

;; - ad-remove-advice removes a particular piece of advice of a FUNCTION.
;;               You still have to do call `ad-activate' or `ad-update' to
;;               activate the new state of advice.
;; - ad-enable-advice enables a particular piece of advice of a FUNCTION.
;; - ad-disable-advice disables a particular piece of advice of a FUNCTION.
;; - ad-enable-regexp maps over all currently advised functions and enables
;;               every advice whose name contains a match for a regular
;;               expression.
;; - ad-disable-regexp disables matching advices.

;; - ad-activate-regexp   activates all advised function with a matching advice
;; - ad-deactivate-regexp deactivates all advised function with matching advice
;; - ad-update-regexp     updates all advised function with a matching advice
;; - ad-activate-all      activates all advised functions
;; - ad-deactivate-all    deactivates all advised functions
;; - ad-update-all        updates all advised functions
;; - ad-unadvise-all      unadvises all advised functions
;; - ad-recover-all       recovers all advised functions

;; - ad-compile byte-compiles a function/macro if it is compilable.

;; @@ Summary of forms with special meanings when used within an advice:
;; =====================================================================
;;   ad-return-value   name of the return value variable (get/settable)
;;   ad-subr-args      name of &rest argument variable used for advised
;;                     subrs whose actual argument list cannot be
;;                     determined (get/settable)
;;   (ad-get-arg <pos>), (ad-get-args <pos>),
;;   (ad-set-arg <pos> <value>), (ad-set-args <pos> <value-list>)
;;                     argument access text macros to get/set the values of
;;                     actual arguments at a certain position
;;   ad-arg-bindings   text macro that returns the actual names, values
;;                     and types of the arguments as a list of bindings. The
;;                     order of the bindings corresponds to the order of the
;;                     arguments. The individual fields of every binding (name,
;;                     value and type) can be accessed with the function
;;                     `ad-arg-binding-field' (see example above).
;;   ad-do-it          text macro that identifies the place where the original
;;                     or wrapped definition should go in an around advice


;; @ Foo games: An advice tutorial
;; ===============================
;; The following tutorial was created in GNU Emacs 18.59. Left-justified
;; s-expressions are input forms followed by one or more result forms.
;; First we have to start the advice magic:
;;
;; (ad-start-advice)
;; nil
;;
;; We start by defining an innocent looking function `foo' that simply
;; adds 1 to its argument X:
;;  
;; (defun foo (x)
;;   "Add 1 to X."
;;   (1+ x))
;; foo
;;
;; (foo 3)
;; 4
;;
;; @@ Defining a simple piece of advice:
;; =====================================
;; Now let's define the first piece of advice for `foo'.  To do that we
;; use the macro `defadvice' which takes a function name, a list of advice
;; specifiers and a list of body forms as arguments.  The first element of
;; the advice specifiers is the class of the advice, the second is its name,
;; the third its position and the rest are some flags. The class of our
;; first advice is `before', its name is `fg-add2', its position among the
;; currently defined before advices (none so far) is `first', and the advice
;; will be `activate'ed immediately. Advice names are global symbols, hence,
;; the name space conventions used for function names should be applied. All
;; advice names in this tutorial will be prefixed with `fg' for `Foo Games'
;; (because everybody has the right to be inconsistent all the function names
;; used in this tutorial do NOT follow this convention).
;;
;; In the body of an advice we can refer to the argument variables of the
;; original function by name. Here we add 1 to X so the effect of calling
;; `foo' will be to actually add 2. All of the advice definitions below only
;; have one body form for simplicity, but there is no restriction to that
;; extent. Every piece of advice can have a documentation string which will
;; be combined with the documentation of the original function.
;;
;; (defadvice foo (before fg-add2 first activate)
;;   "Add 2 to X."
;;   (setq x (1+ x)))
;; foo
;;
;; (foo 3)
;; 5
;;
;; @@ Specifying the position of an advice:
;; ========================================
;; Now we define the second before advice which will cancel the effect of
;; the previous advice. This time we specify the position as 0 which is
;; equivalent to `first'. A number can be used to specify the zero-based
;; position of an advice among the list of advices in the same class. This
;; time we already have one before advice hence the position specification
;; actually has an effect. So, after the following definition the position
;; of the previous advice will be 1 even though we specified it with `first'
;; above, the reason for this is that the position argument is relative to
;; the currently defined pieces of advice which by now has changed.
;;
;; (defadvice foo (before fg-cancel-add2 0 activate)
;;   "Again only add 1 to X."
;;   (setq x (1- x)))
;; foo
;;
;; (foo 3)
;; 4
;;
;; @@ Redefining a piece of advice:
;; ================================
;; Now we define an advice with the same class and same name but with a
;; different position. Defining an advice in a class in which an advice with
;; that name already exists is interpreted as a redefinition of that
;; particular advice, in which case the position argument will be ignored
;; and the previous position of the redefined piece of advice is used.
;; Advice flags can be specified with non-ambiguous initial substrings, hence,
;; from now on we'll use `act' instead of the verbose `activate'.
;;
;; (defadvice foo (before fg-cancel-add2 last act)
;;   "Again only add 1 to X."
;;   (setq x (1- x)))
;; foo
;;
;; @@ Assembly of advised documentation:
;; =====================================
;; The documentation strings of the various pieces of advice are assembled
;; in order which shows that advice `fg-cancel-add2' is still the first
;; `before' advice even though we specified position `last' above:
;;
;; (documentation 'foo)
;; "Add 1 to X.
;;
;; This function is advised with the following advice(s):
;;
;; fg-cancel-add2 (before):
;; Again only add 1 to X.
;;
;; fg-add2 (before):
;; Add 2 to X."
;;
;; @@ Advising interactive behavior:
;; =================================
;; We can make a function interactive (or change its interactive behavior)
;; by specifying an interactive form in one of the before or around
;; advices (there could also be body forms in this advice). The particular
;; definition always assigns 5 as an argument to X which gives us 6 as a
;; result when we call foo interactively:
;;
;; (defadvice foo (before fg-inter last act)
;;   "Use 5 as argument when called interactively."
;;   (interactive (list 5)))
;; foo
;;
;; (call-interactively 'foo)
;; 6
;;
;; If more than one advice have an interactive declaration, then the one of
;; the advice with the smallest position will be used (before advices go
;; before around and after advices), hence, the declaration below does
;; not have any effect:
;;
;; (defadvice foo (before fg-inter2 last act)
;;   (interactive (list 6)))
;; foo
;;
;; (call-interactively 'foo)
;; 6
;;
;; Let's have a look at what the definition of `foo' looks like now 
;; (indentation added by hand for legibility):
;;
;; (symbol-function 'foo)
;; (lambda (x)
;;   "$ad-doc: foo$"
;;   (interactive (list 5))
;;   (let (ad-return-value) 
;;     (setq x (1- x)) 
;;     (setq x (1+ x)) 
;;     (setq ad-return-value (ad-Orig-foo x)) 
;;     ad-return-value))
;;
;; @@ Around advices:
;; ==================
;; Now we'll try some `around' advices. An around advice is a wrapper around
;; the original definition. It can shadow or establish bindings for the
;; original definition, and it can look at and manipulate the value returned
;; by the original function. The position of the special keyword `ad-do-it'
;; specifies where the code of the original function will be executed. The
;; keyword can appear multiple times which will result in multiple calls of
;; the original function in the resulting advised code. Note, that if we don't
;; specify a position argument (i.e., `first', `last' or a number), then 
;; `first' (or 0) is the default):
;;
;; (defadvice foo (around fg-times-2 act)
;;   "First double X."
;;   (let ((x (* x 2)))
;;     ad-do-it))
;; foo
;;
;; (foo 3)
;; 7
;;
;; Around advices are assembled like onion skins where the around advice
;; with position 0 is the outermost skin and the advice at the last position
;; is the innermost skin which is directly wrapped around the call of the
;; original definition of the function. Hence, after the next `defadvice' we
;; will first multiply X by 2 then add 1 and then call the original
;; definition (i.e., add 1 again):
;;
;; (defadvice foo (around fg-add-1 last act)
;;   "Add 1 to X."
;;   (let ((x (1+ x)))
;;     ad-do-it))
;; foo
;;
;; (foo 3)
;; 8
;;
;; Again, let's see what the definition of `foo' looks like so far:
;;
;; (symbol-function 'foo)
;; (lambda (x) 
;;   "$ad-doc: foo$"
;;   (interactive (list 5)) 
;;   (let (ad-return-value) 
;;     (setq x (1- x)) 
;;     (setq x (1+ x)) 
;;     (let ((x (* x 2))) 
;;       (let ((x (1+ x))) 
;;         (setq ad-return-value (ad-Orig-foo x)))) 
;;     ad-return-value))
;;
;; @@ Controlling advice activation:
;; =================================
;; In every `defadvice' so far we have used the flag `activate' to activate
;; the advice immediately after its definition, and that's what we want in
;; most cases. However, if we define multiple pieces of advice for a single
;; function then activating every advice immediately is inefficient. A
;; better way to do this is to only activate the last defined advice.
;; For example:
;;
;; (defadvice foo (after fg-times-x)
;;   "Multiply the result with X."
;;   (setq ad-return-value (* ad-return-value x)))
;; foo
;;
;; This still yields the same result as before:
;; (foo 3)
;; 8
;;
;; Now we define another advice and activate which will also activate the
;; previous advice `fg-times-x'. Note the use of the special variable
;; `ad-return-value' in the body of the advice which is set to the result of
;; the original function. If we change its value then the value returned by
;; the advised function will be changed accordingly:
;;
;; (defadvice foo (after fg-times-x-again act)
;;   "Again multiply the result with X."
;;   (setq ad-return-value (* ad-return-value x)))
;; foo
;;
;; Now the advices have an effect:
;;
;; (foo 3)
;; 72
;;
;; @@ Protecting advice execution:
;; ===============================
;; Once in a while we define an advice to perform some cleanup action, 
;; for example:
;;
;; (defadvice foo (after fg-cleanup last act)
;;   "Do some cleanup."
;;   (print "Let's clean up now!"))
;; foo
;;
;; However, in case of an error the cleanup won't be performed:
;;
;; (condition-case error
;;     (foo t)
;;   (error 'error-in-foo))
;; error-in-foo
;;
;; To make sure a certain piece of advice gets executed even if some error or
;; non-local exit occurred in any preceding code, we can protect it by using
;; the `protect' keyword. (if any of the around advices is protected then the
;; whole around advice onion will be protected):
;;
;; (defadvice foo (after fg-cleanup prot act)
;;   "Do some protected cleanup."
;;   (print "Let's clean up now!"))
;; foo
;;
;; Now the cleanup form will be executed even in case of an error:
;;
;; (condition-case error
;;     (foo t)
;;   (error 'error-in-foo))
;; "Let's clean up now!"
;; error-in-foo
;;
;; Again, let's see what `foo' looks like:
;;
;; (symbol-function 'foo)
;; (lambda (x) 
;;   "$ad-doc: foo$"
;;   (interactive (list 5)) 
;;   (let (ad-return-value) 
;;     (unwind-protect 
;;         (progn (setq x (1- x)) 
;;                (setq x (1+ x)) 
;;                (let ((x (* x 2))) 
;;                  (let ((x (1+ x))) 
;;                    (setq ad-return-value (ad-Orig-foo x)))) 
;;                (setq ad-return-value (* ad-return-value x)) 
;;                (setq ad-return-value (* ad-return-value x))) 
;;       (print "Let's clean up now!")) 
;;     ad-return-value))
;;
;; @@ Compilation of advised definitions:
;; ======================================
;; Finally, we can specify the `compile' keyword in a `defadvice' to say
;; that we want the resulting advised function to be byte-compiled
;; (`compile' will be ignored unless we also specified `activate'):
;;
;; (defadvice foo (after fg-cleanup prot act comp)
;;   "Do some protected cleanup."
;;   (print "Let's clean up now!"))
;; foo
;;
;; Now `foo' is byte-compiled:
;;
;; (symbol-function 'foo)
;; (lambda (x) 
;;   "$ad-doc: foo$"
;;   (interactive (byte-code "....." [5] 1)) 
;;   (byte-code "....." [ad-return-value x nil ((byte-code "....." [print "Let's clean up now!"] 2)) * 2 ad-Orig-foo] 6))
;;
;; (foo 3)
;; "Let's clean up now!"
;; 72
;;
;; @@ Enabling and disabling pieces of advice:
;; ===========================================
;; Once in a while it is desirable to temporarily disable a piece of advice
;; so that it won't be considered during activation, for example, if two
;; different packages advise the same function and one wants to temporarily
;; neutralize the effect of the advice of one of the packages.
;;
;; The following disables the after advice `fg-times-x' in the function `foo'.
;; All that does is to change a flag for this particular advice. All the
;; other information defining it will be left unchanged (e.g., its relative
;; position in this advice class, etc.).
;;
;; (ad-disable-advice 'foo 'after 'fg-times-x)
;; nil
;;
;; For this to have an effect we have to activate `foo':
;;
;; (ad-activate 'foo)
;; foo
;;
;; (foo 3)
;; "Let's clean up now!"
;; 24
;;
;; If we want to disable all multiplication advices in `foo' we can use a
;; regular expression that matches the names of such advices. Actually, any
;; advice name that contains a match for the regular expression will be
;; called a match. A special advice class `any' can be used to consider
;; all advice classes:
;;
;; (ad-disable-advice 'foo 'any "^fg-.*times")
;; nil
;;
;; (ad-activate 'foo)
;; foo
;;
;; (foo 3)
;; "Let's clean up now!"
;; 5
;;
;; To enable the disabled advice we could use either `ad-enable-advice'
;; similar to `ad-disable-advice', or as an alternative `ad-enable-regexp'
;; which will enable matching advices in ALL currently advised functions.
;; Hence, this can be used to dis/enable advices made by a particular
;; package to a set of functions as long as that package obeys standard
;; advice name conventions.  We prefixed all advice names with `fg-', hence
;; the following will do the trick (`ad-enable-regexp' returns the number
;; of matched advices):
;;
;; (ad-enable-regexp "^fg-")
;; 9
;;
;; The following will activate all currently active advised functions that
;; contain some advice matched by the regular expression. This is a save
;; way to update the activation of advised functions whose advice changed
;; in some way or other without accidentally also activating currently
;; deactivated functions:
;;
;; (ad-update-regexp "^fg-")
;; nil
;;
;; (foo 3)
;; "Let's clean up now!"
;; 72
;;
;; Another use for the dis/enablement mechanism is to define a piece of advice
;; and keep it "dormant" until a particular condition is satisfied, i.e., until
;; then the advice will not be used during activation. The `disable' flag lets
;; one do that with `defadvice':
;;
;; (defadvice foo (before fg-1-more dis)
;;   "Add yet 1 more."
;;   (setq x (1+ x)))
;; foo
;;
;; (ad-activate 'foo)
;; foo
;;
;; (foo 3)
;; "Let's clean up now!"
;; 72
;;
;; (ad-enable-advice 'foo 'before 'fg-1-more)
;; nil
;;
;; (ad-activate 'foo)
;; foo
;;
;; (foo 3)
;; "Let's clean up now!"
;; 160
;;
;; @@ Caching:
;; ===========
;; Advised definitions get cached to allow efficient activation/deactivation
;; without having to reconstruct them if nothing in the advice-info of a
;; function has changed. The following idiom can be used to temporarily
;; deactivate functions that have a piece of advice defined by a certain
;; package (we save the old definition to check out caching):
;;
;; (setq old-definition (symbol-function 'foo))
;; (lambda (x) ....)
;;
;; (ad-deactivate-regexp "^fg-")
;; nil
;;
;; (foo 3)
;; 4
;;
;; (ad-activate-regexp "^fg-")
;; nil
;;
;; (eq old-definition (symbol-function 'foo))
;; t
;;
;; (foo 3)
;; "Let's clean up now!"
;; 160
;;
;; @@ Forward advice:
;; ==================
;; To enable automatic activation of forward advice we first have to set
;; `ad-activate-on-definition' to t and restart advice:
;;
;; (setq ad-activate-on-definition t)
;; t
;;
;; (ad-start-advice)
;; (ad-activate-defined-function)
;;
;; Let's define a piece of advice for an undefined function:
;;
;; (defadvice bar (before fg-sub-1-more act)
;;   "Subtract one more from X."
;;   (setq x (1- x)))
;; bar
;;
;; `bar' is not yet defined:
;; (fboundp 'bar)
;; nil
;;
;; Now we define it and the forward advice will get activated (only because
;; `ad-activate-on-definition' was t when we started advice above with
;; `ad-start-advice'):
;;
;; (defun bar (x)
;;   "Subtract 1 from X."
;;   (1- x))
;; bar
;;
;; (bar 4)
;; 2
;;
;; Redefinition will activate any available advice if the value of
;; `ad-redefinition-action' is either `warn', `accept' or `discard':
;;
;; (defun bar (x)
;;   "Subtract 2 from X."
;;   (- x 2))
;; bar
;;
;; (bar 4)
;; 1
;;
;; @@ Preactivation:
;; =================
;; Constructing advised definitions is moderately expensive, hence, it is
;; desirable to have a way to construct them at byte-compile time.
;; Preactivation is a mechanism that allows one to do that.
;;
;; (defun fie (x)
;;   "Multiply X by 2."
;;   (* x 2))
;; fie
;;
;; (defadvice fie (before fg-times-4 preact)
;;   "Multiply X by 4."
;;   (setq x (* x 2)))
;; fie
;;
;; This advice did not affect `fie'...
;;
;; (fie 2)
;; 4
;;
;; ...but it constructed a cached definition that will be used once `fie' gets
;; activated as long as its current advice state is the same as it was during
;; preactivation:
;;
;; (setq cached-definition (ad-get-cache-definition 'fie))
;; (lambda (x) ....)
;;
;; (ad-activate 'fie)
;; fie
;;
;; (eq cached-definition (symbol-function 'fie))
;; t
;;
;; (fie 2)
;; 8
;;
;; If you put a preactivating `defadvice' into an elisp file that gets byte-
;; compiled then the constructed advised definition will get compiled by
;; the byte-compiler. For that to occur in a v18 emacs you have to put the
;; `defadvice' inside a `defun' because the v18 compiler does not compile
;; top-level forms other than `defun' or `defmacro', for example,
;;
;; (defun fg-defadvice-fum ()
;;   (defadvice fum (before fg-times-4 preact act)
;;     "Multiply X by 4."
;;     (setq x (* x 2))))
;; fg-defadvice-fum
;;
;; So far, no `defadvice' for `fum' got executed, but when we compile
;; `fg-defadvice-fum' the `defadvice' will be expanded by the byte compiler.
;; In order for preactivation to be effective we have to have a proper
;; definition of `fum' around at preactivation time, hence, we define it now:
;;
;; (defun fum (x)
;;   "Multiply X by 2."
;;   (* x 2))
;; fum
;;
;; Now we compile the defining function which will construct an advised
;; definition during expansion of the `defadvice', compile it and store it
;; as part of the compiled `fg-defadvice-fum':
;;
;; (ad-compile-function 'fg-defadvice-fum)
;; (lambda nil (byte-code ...))
;;
;; `fum' is still completely unaffected:
;;
;; (fum 2)
;; 4
;;
;; (ad-get-advice-info 'fum)
;; nil
;;
;; (fg-defadvice-fum)
;; fum
;;
;; Now the advised version of `fum' is compiled because the compiled definition
;; constructed during preactivation was used, even though we did not specify
;; the `compile' flag:
;;
;; (symbol-function 'fum)
;; (lambda (x) 
;;   "$ad-doc: fum$"
;;   (byte-code "....." [ad-return-value x nil * 2 ad-Orig-fum] 4))
;;
;; (fum 2)
;; 8
;;
;; A preactivated definition will only be used if it matches the current
;; function definition and advice information. If it does not match it
;; will simply be discarded and a new advised definition will be constructed
;; from scratch. For example, let's first remove all advice-info for `fum':
;;
;; (ad-unadvise 'fum)
;; (("fie") ("bar") ("foo") ...)
;;
;; And now define a new piece of advice:
;;
;; (defadvice fum (before fg-interactive act)
;;   "Make fum interactive."
;;   (interactive "nEnter x: "))
;; fum
;;
;; When we now try to use a preactivation it will not be used because the
;; current advice state is different from the one at preactivation time. This
;; is no tragedy, everything will work as expected just not as efficient,
;; because a new advised definition has to be constructed from scratch:
;;
;; (fg-defadvice-fum)
;; fum
;;
;; A new uncompiled advised definition got constructed:
;;
;; (ad-compiled-p (symbol-function 'fum))
;; nil
;;
;; (fum 2)
;; 8
;;
;; MORAL: To get all the efficiency out of preactivation the function
;; definition and advice state at preactivation time must be the same as the
;; state at activation time. Preactivation does work with forward advice, all
;; that's necessary is that the definition of the forward advised function is
;; available when the `defadvice' with the preactivation gets compiled.
;;
;; @@ Portable argument access:
;; ============================
;; So far, we always used the actual argument variable names to access an
;; argument in a piece of advice. For many advice applications this is
;; perfectly ok and keeps advices simple. However, it decreases portability
;; of advices because it assumes specific argument variable names. For example,
;; if one advises a subr such as `eval-region' which then gets redefined by
;; some package (e.g., edebug) into a function with different argument names,
;; then a piece of advice written for `eval-region' that was written with
;; the subr arguments in mind will break. Similar situations arise when one
;; switches between major Emacs versions, e.g., certain subrs in v18 are
;; functions in v19 and vice versa. Also, in v19s subr argument lists
;; are available and will be used, while they are not available in v18.
;;
;; Argument access text macros allow one to access arguments of an advised
;; function in a portable way without having to worry about all these
;; possibilities. These macros will be translated into the proper access forms
;; at activation time, hence, argument access will be as efficient as if
;; the arguments had been used directly in the definition of the advice.
;;
;; (defun fuu (x y z)
;;   "Add 3 numbers."
;;   (+ x y z))
;; fuu
;;
;; (fuu 1 1 1)
;; 3
;;
;; Argument access macros specify actual arguments at a certain position.
;; Position 0 access the first actual argument, position 1 the second etc.
;; For example, the following advice adds 1 to each of the 3 arguments:
;;
;; (defadvice fuu (before fg-add-1-to-all act)
;;   "Adds 1 to all arguments."
;;   (ad-set-arg 0 (1+ (ad-get-arg 0)))
;;   (ad-set-arg 1 (1+ (ad-get-arg 1)))
;;   (ad-set-arg 2 (1+ (ad-get-arg 2))))
;; fuu
;;
;; (fuu 1 1 1)
;; 6
;;
;; Now suppose somebody redefines `fuu' with a rest argument. Our advice
;; will still work because we used access macros (note, that automatic
;; advice activation is still in effect, hence, the redefinition of `fuu'
;; will automatically activate all its advice):
;;
;; (defun fuu (&rest numbers)
;;   "Add NUMBERS."
;;   (apply '+ numbers))
;; fuu
;;
;; (fuu 1 1 1)
;; 6
;;
;; (fuu 1 1 1 1 1 1)
;; 9
;;
;; What's important to notice is that argument access macros access actual
;; arguments regardless of how they got distributed onto argument variables.
;; In Emacs Lisp the semantics of an actual argument is determined purely
;; by position, hence, as long as nobody changes the semantics of what a
;; certain actual argument at a certain position means the access macros
;; will do the right thing.
;;
;; Because of &rest arguments we need a second kind of access macro that
;; can access all actual arguments starting from a certain position:
;;
;; (defadvice fuu (before fg-print-args act)
;;   "Print all arguments."
;;   (print (ad-get-args 0)))
;; fuu
;;
;; (fuu 1 2 3 4 5)
;; (1 2 3 4 5)
;; 18
;;
;; (defadvice fuu (before fg-set-args act)
;;   "Swaps 2nd and 3rd arg and discards all the rest."
;;   (ad-set-args 1 (list (ad-get-arg 2) (ad-get-arg 1))))
;; fuu
;;
;; (fuu 1 2 3 4 4 4 4 4 4)
;; (1 3 2)
;; 9
;;
;; (defun fuu (x y z)
;;   "Add 3 numbers."
;;   (+ x y z))
;;
;; (fuu 1 2 3)
;; (1 3 2)
;; 9
;;
;; @@ Defining the argument list of an advised function:
;; =====================================================
;; Once in a while it might be desirable to advise a function and additionally
;; give it an extra argument that controls the advised code, for example, one
;; might want to make an interactive function sensitive to a prefix argument.
;; For such cases `defadvice' allows the specification of an argument list
;; for the advised function. Similar to the redefinition of interactive 
;; behavior, the first argument list specification found in the list of before/
;; around/after advices will be used. Of course, the specified argument list
;; should be downward compatible with the original argument list, otherwise
;; functions that call the advised function with the original argument list
;; in mind will break.
;;
;; (defun fii (x)
;;   "Add 1 to X."
;;   (1+ x))
;; fii
;;
;; Now we advise `fii' to use an optional second argument that controls the
;; amount of incrementation. A list following the (optional) position
;; argument of the advice will be interpreted as an argument list
;; specification. This means you cannot specify an empty argument list, and
;; why would you want to anyway?
;;
;; (defadvice fii (before fg-inc-x (x &optional incr) act)
;;   "Increment X by INCR (default is 1)."
;;   (setq x (+ x (1- (or incr 1)))))
;; fii
;;
;; (fii 3)
;; 4
;;
;; (fii 3 2)
;; 5
;;
;; @@ Specifying argument lists of subrs:
;; ======================================
;; The argument lists of subrs cannot be determined directly from Lisp.
;; This means that advice.el has to use `(&rest ad-subr-args)' as the
;; argument list of the advised subr which is not very efficient. In Lemacs
;; subr argument lists can be determined from their documentation string, in
;; GNU Emacs-19 this is the case for some but not all subrs. To accommodate
;; for the cases where the argument lists cannot be determined (e.g., in a
;; v18 Emacs) advice.el comes with a specification mechanism that allows the
;; advice programmer to tell advice what the argument list of a certain subr
;; really is.
;;
;; In a v18 Emacs the following will return the &rest idiom:
;;
;; (ad-arglist (symbol-function 'car))
;; (&rest ad-subr-args)
;;
;; To tell advice what the argument list of `car' really is we
;; can do the following:
;;
;; (ad-define-subr-args 'car '(list))
;; ((list))
;;
;; Now `ad-arglist' will return the proper argument list (this method is
;; actually used by advice itself for the advised definition of `fset'):
;;
;; (ad-arglist (symbol-function 'car))
;; (list)
;;
;; The defined argument list will be stored on the property list of the
;; subr name symbol. When advice looks for a subr argument list it first
;; checks for a definition on the property list, if that fails it tries
;; to infer it from the documentation string and caches it on the property
;; list if it was successful, otherwise `(&rest ad-subr-args)' will be used.
;;
;; @@ Advising interactive subrs:
;; ==============================
;; For the most part there is no difference between advising functions and
;; advising subrs. There is one situation though where one might have to write
;; slightly different advice code for subrs than for functions. This case
;; arises when one wants to access subr arguments in a before/around advice
;; when the arguments were determined by an interactive call to the subr.
;; Advice cannot determine what `interactive' form determines the interactive
;; behavior of the subr, hence, when it calls the original definition in an
;; interactive subr invocation it has to use `call-interactively' to generate
;; the proper interactive behavior. Thus up to that call the arguments of the
;; interactive subr will be nil. For example, the following advice for
;; `kill-buffer' will not work in an interactive invocation...
;;
;; (defadvice kill-buffer (before fg-kill-buffer-hook first act preact comp)
;;   (my-before-kill-buffer-hook (ad-get-arg 0)))
;; kill-buffer
;;
;; ...because the buffer argument will be nil in that case. The way out of
;; this dilemma is to provide an `interactive' specification that mirrors
;; the interactive behavior of the unadvised subr, for example, the following
;; will do the right thing even when `kill-buffer' is called interactively:
;;
;; (defadvice kill-buffer (before fg-kill-buffer-hook first act preact comp)
;;   (interactive "bKill buffer: ")
;;   (my-before-kill-buffer-hook (ad-get-arg 0)))
;; kill-buffer
;;
;; @@ Advising macros:
;; ===================
;; Advising macros is slightly different because there are two significant
;; time points in the invocation of a macro: Expansion and evaluation time.
;; For an advised macro instead of evaluating the original definition we
;; use `macroexpand', that is, changing argument values and binding
;; environments by pieces of advice has an affect during macro expansion
;; but not necessarily during evaluation. In particular, any side effects
;; of pieces of advice will occur during macro expansion.  To also affect
;; the behavior during evaluation time one has to change the value of
;; `ad-return-value' in a piece of after advice. For example:
;;
;; (defmacro foom (x)
;;   (` (list (, x))))
;; foom
;;
;; (foom '(a))
;; ((a))
;;
;; (defadvice foom (before fg-print-x act)
;;   "Print the value of X."
;;   (print x))
;; foom
;;
;; The following works as expected because evaluation immediately follows
;; macro expansion:
;;
;; (foom '(a))
;; (quote (a))
;; ((a))
;;
;; However, the printing happens during expansion (or byte-compile) time:
;;
;; (macroexpand '(foom '(a)))
;; (quote (a))
;; (list (quote (a)))
;;
;; If we want it to happen during evaluation time we have to do the 
;; following (first remove the old advice):
;;
;; (ad-remove-advice 'foom 'before 'fg-print-x)
;; nil
;;
;; (defadvice foom (after fg-print-x act)
;;   "Print the value of X."
;;   (setq ad-return-value
;;         (` (progn (print (, x))
;;                   (, ad-return-value)))))
;; foom
;;
;; (macroexpand '(foom '(a)))
;; (progn (print (quote (a))) (list (quote (a))))
;;
;; (foom '(a))
;; (a)
;; ((a))
;;
;; While this method might seem somewhat cumbersome, it is very general
;; because it allows one to influence macro expansion as well as evaluation.
;; In general, advising macros should be a rather rare activity anyway, in
;; particular, because compile-time macro expansion takes away a lot of the
;; flexibility and effectiveness of the advice mechanism. Macros that were
;; compile-time expanded before the advice was activated will of course never
;; exhibit the advised behavior.
;;
;; @@ Advising special forms:
;; ==========================
;; Now for something that should be even more rare than advising macros:
;; Advising special forms. Because special forms are irregular in their
;; argument evaluation behavior (e.g., `setq' evaluates the second but not
;; the first argument) they have to be advised into macros. A dangerous
;; consequence of this is that the byte-compiler will not recognize them
;; as special forms anymore (well, in most cases) and use their expansion
;; rather than the proper byte-code. Also, because the original definition
;; of a special form cannot be `funcall'ed, `eval' has to be used instead
;; which is less efficient.
;;
;; MORAL: Do not advise special forms unless you are completely sure about
;;        what you are doing (some of the forward advice behavior is
;;        implemented via advice of the special forms `defun' and `defmacro').
;;        As a safety measure one should always do `ad-deactivate-all' before
;;        one byte-compiles a file to avoid any interference of advised
;;        special forms.
;;
;; Apart from the safety concerns advising special forms is not any different
;; from advising plain functions or subrs.


;;; Change Log:

;; advice.el,v
;; Revision 2.1  1993/05/26  00:07:58  hans
;; 	* advise `defalias' and `define-function' to properly handle forward
;; 	  advice in GNU Emacs-19.7 and later
;; 	* fix minor bug in `ad-preactivate-advice'
;; 	* merge with FSF installation of version 2.0
;;
;; Revision 2.0 1993/05/18 01:29:02 hans
;;	* Totally revamped: Now also works with v19s, function indirection
;;	  instead of body copying for original function calls, caching of
;;	  advised definitions, en/disable mechanism, more and better
;;        interactive functions, forward advice support for jwz's compiler,
;;        definition hooks, portable argument access, argument list definition
;;        for advised functions, preactivation mechanism, pretty comprehensive
;;        docs (still no info file)
;;
;; Revision 1.8 1992/12/15 22:54:45 hans
;;	* Replaced non-standard `member' with `memq'.
;;
;; Revision 1.7 1992/12/14 22:41:49 hans
;;	* First publicly released version
;;
;; Revision 1.1 1992/12/12 05:37:33 hans
;;	* Created


;;; Code:

;; @ Advice implementation:
;; ========================

;; @@ Compilation idiosyncrasies:
;; ==============================

;; `defadvice' expansion needs quite a few advice functions and variables,
;; hence, I need to preload the file before it can be compiled. To avoid
;; interference of bogus compiled files I always preload the source file:
(provide 'advice-preload)
;; During a normal load this is a noop:
(require 'advice-preload "advice.el")

;; For the odd case that ``' does not have an autoload definition in some
;; Emacs we autoload it here. It is only needed for compilation, hence,
;; I don't want to unconditionally `require' it (re-autoloading ``' after
;; this file got preloaded will properly redefine this autoload):
(if (not (fboundp '`)) (autoload '` "backquote"))


;; @@ Variable definitions:
;; ========================

(defconst ad-version "2.1")

(defconst ad-emacs19-p
  (not (or (and (boundp 'epoch::version) epoch::version)
	   (string-lessp emacs-version "19")))
  "Non-NIL if we run Emacs version 19 or higher.
This will be true for GNU Emacs-19 as well as Lemacs.")

(defconst ad-lemacs-p
  (and ad-emacs19-p (string-match "Lucid" emacs-version))
  "Non-NIL if we run Lucid's version of Emacs-19.")

;;;###autoload
2034
(defvar ad-start-advice-on-load t
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