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;;; help.el --- help commands for Emacs

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;; Copyright (C) 1985, 1986, 1993, 1994, 1998 Free Software Foundation, Inc.
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;; Maintainer: FSF
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;; Keywords: help, internal
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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
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;; the Free Software Foundation; either version 2, or (at your option)
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;; 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
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;; along with GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.
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;;; Commentary:

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;; This code implements GNU Emacs' on-line help system, the one invoked by
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;;`M-x help-for-help'.

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;;; Code:

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;; Get the macro make-help-screen when this is compiled,
;; or run interpreted, but not when the compiled code is loaded.
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(eval-when-compile (require 'help-macro))
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(defvar help-map (make-sparse-keymap)
  "Keymap for characters following the Help key.")

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(defvar help-mode-map (make-sparse-keymap)
  "Keymap for help mode.")

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(define-key global-map (char-to-string help-char) 'help-command)
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(define-key global-map [help] 'help-command)
(define-key global-map [f1] 'help-command)
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(fset 'help-command help-map)

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(define-key help-map (char-to-string help-char) 'help-for-help)
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(define-key help-map [help] 'help-for-help)
(define-key help-map [f1] 'help-for-help)
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(define-key help-map "?" 'help-for-help)

(define-key help-map "\C-c" 'describe-copying)
(define-key help-map "\C-d" 'describe-distribution)
(define-key help-map "\C-w" 'describe-no-warranty)
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(define-key help-map "\C-p" 'describe-project)
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(define-key help-map "a" 'apropos-command)
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(define-key help-map "b" 'describe-bindings)

(define-key help-map "c" 'describe-key-briefly)
(define-key help-map "k" 'describe-key)

(define-key help-map "d" 'describe-function)
(define-key help-map "f" 'describe-function)

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(define-key help-map "F" 'view-emacs-FAQ)

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(define-key help-map "i" 'info)
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(define-key help-map "\C-f" 'Info-goto-emacs-command-node)
(define-key help-map "\C-k" 'Info-goto-emacs-key-command-node)
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(define-key help-map "\C-i" 'info-lookup-symbol)
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(define-key help-map "l" 'view-lossage)

(define-key help-map "m" 'describe-mode)

(define-key help-map "\C-n" 'view-emacs-news)
(define-key help-map "n" 'view-emacs-news)

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(define-key help-map "p" 'finder-by-keyword)
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(autoload 'finder-by-keyword "finder"
  "Find packages matching a given keyword." t)
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(define-key help-map "s" 'describe-syntax)

(define-key help-map "t" 'help-with-tutorial)

(define-key help-map "w" 'where-is)

(define-key help-map "v" 'describe-variable)

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(define-key help-map "q" 'help-quit)

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(define-key help-mode-map [mouse-2] 'help-follow-mouse)
(define-key help-mode-map "\C-c\C-b" 'help-go-back)
(define-key help-mode-map "\C-c\C-c" 'help-follow)
(define-key help-mode-map "\t" 'help-next-ref)
(define-key help-mode-map [backtab] 'help-previous-ref)
;; Documentation only, since we use minor-mode-overriding-map-alist.
(define-key help-mode-map "\r" 'help-follow)

;; Font-locking is incompatible with the new xref stuff.
;(defvar help-font-lock-keywords
;  (eval-when-compile
;    (let ((name-char "[-+a-zA-Z0-9_*]") (sym-char "[-+a-zA-Z0-9_:*]"))
;      (list
;       ;;
;       ;; The symbol itself.
;       (list (concat "\\`\\(" name-char "+\\)\\(\\(:\\)\\|\\('\\)\\)")
;	     '(1 (if (match-beginning 3)
;		     font-lock-function-name-face
;		   font-lock-variable-name-face)))
;       ;;
;       ;; Words inside `' which tend to be symbol names.
;       (list (concat "`\\(" sym-char sym-char "+\\)'")
;	     1 'font-lock-constant-face t)
;       ;;
;       ;; CLisp `:' keywords as references.
;       (list (concat "\\<:" sym-char "+\\>") 0 'font-lock-builtin-face t))))
;  "Default expressions to highlight in Help mode.")

(defvar help-xref-stack nil
  "A stack of ways by which to return to help buffers after following xrefs.
Used by `help-follow' and `help-xref-go-back'.")
(put 'help-xref-stack 'permanent-local t)

(defvar help-xref-stack-item nil
  "An item for `help-follow' in this buffer to push onto `help-xref-stack'.")
(put 'help-xref-stack-item 'permanent-local t)

(setq-default help-xref-stack nil help-xref-stack-item nil)
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(defun help-mode ()
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  "Major mode for viewing help text and navigating references in it.
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Entry to this mode runs the normal hook `help-mode-hook'.
Commands:
\\{help-mode-map}"
  (interactive)
  (kill-all-local-variables)
  (use-local-map help-mode-map)
  (setq mode-name "Help")
  (setq major-mode 'help-mode)
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  (make-local-variable 'font-lock-defaults)
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  (setq font-lock-defaults nil)         ; font-lock would defeat xref
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  (view-mode)
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  (make-local-variable 'view-no-disable-on-exit)
  (setq view-no-disable-on-exit t)
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  ;; `help-make-xrefs' would be run here if not invoked from
  ;; `help-mode-maybe'.
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  (run-hooks 'help-mode-hook))

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(defun help-mode-maybe ()
  (if (eq major-mode 'fundamental-mode)
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      (help-mode))
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  (when (eq major-mode 'help-mode) 
    ;; View mode's read-only status of existing *Help* buffer is lost
    ;; by with-output-to-temp-buffer.
    (toggle-read-only 1)
    (help-make-xrefs (current-buffer)))
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  (setq view-return-to-alist
	(list (cons (selected-window) help-return-method))))
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(add-hook 'temp-buffer-show-hook 'help-mode-maybe)

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(defun help-quit ()
  (interactive)
  nil)

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(defun help-with-tutorial (&optional arg)
  "Select the Emacs learn-by-doing tutorial.
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If there is a tutorial version written in the language
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of the selected language environment, that version is used.
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If there's no tutorial in that language, `TUTORIAL' is selected.
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With arg, you are asked to choose which language."
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  (interactive "P")
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  (let ((lang (if arg
		  (read-language-name 'tutorial "Language: " "English")
		(if (get-language-info current-language-environment 'tutorial)
		    current-language-environment
		  "English"))))
    (setq filename (get-language-info lang 'tutorial))
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    (setq file (expand-file-name (concat "~/" filename)))
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    (delete-other-windows)
    (if (get-file-buffer file)
	(switch-to-buffer (get-file-buffer file))
      (switch-to-buffer (create-file-buffer file))
      (setq buffer-file-name file)
      (setq default-directory (expand-file-name "~/"))
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      (setq buffer-auto-save-file-name nil)
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      (insert-file-contents (expand-file-name filename data-directory))
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      (goto-char (point-min))
      (search-forward "\n<<")
      (beginning-of-line)
      (delete-region (point) (progn (end-of-line) (point)))
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      (let ((n (- (window-height (selected-window))
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		  (count-lines (point-min) (point))
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		  6)))
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	(if (< n 12)
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	    (newline n)
	  ;; Some people get confused by the large gap.
	  (newline (/ n 2))
	  (insert "[Middle of page left blank for didactic purposes.  "
		  "Text continues below]")
	  (newline (- n (/ n 2)))))
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      (goto-char (point-min))
      (set-buffer-modified-p nil))))

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(defun describe-key-briefly (key &optional insert)
  "Print the name of the function KEY invokes.  KEY is a string.
If INSERT (the prefix arg) is non-nil, insert the message in the buffer."
  (interactive "kDescribe key briefly: \nP")
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  ;; If this key seq ends with a down event, discard the
  ;; following click or drag event.  Otherwise that would
  ;; erase the message.
  (let ((type (aref key (1- (length key)))))
    (if (listp type) (setq type (car type)))
    (and (symbolp type)
	 (memq 'down (event-modifiers type))
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	 (read-event)))
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  (save-excursion
    (let ((modifiers (event-modifiers (aref key 0)))
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	  (standard-output (if insert (current-buffer) t))
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	  window position)
      ;; For a mouse button event, go to the button it applies to
      ;; to get the right key bindings.  And go to the right place
      ;; in case the keymap depends on where you clicked.
      (if (or (memq 'click modifiers) (memq 'down modifiers)
	      (memq 'drag modifiers))
	  (setq window (posn-window (event-start (aref key 0)))
		position (posn-point (event-start (aref key 0)))))
      (if (windowp window)
	  (progn
	    (set-buffer (window-buffer window))
	    (goto-char position)))
      ;; Ok, now look up the key and name the command.
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      (let ((defn (key-binding key))
	    (key-desc (key-description key)))
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	(if (or (null defn) (integerp defn))
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	    (princ (format "%s is undefined" key-desc))
	  (princ (format (if insert
			     "%s (%s)"
			   (if (windowp window)
			       "%s at that spot runs the command %s"
			     "%s runs the command %s"))
			 key-desc
			 (if (symbolp defn) defn (prin1-to-string defn)))))))))
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(defvar help-return-method nil
  "What to do to \"exit\" the help buffer.
This is a list
 (WINDOW . t)              delete the selected window, go to WINDOW.
 (WINDOW . quit-window)    do quit-window, then select WINDOW.
 (WINDOW BUF START POINT)  display BUF at START, POINT, then select WINDOW.")

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(defun print-help-return-message (&optional function)
  "Display or return message saying how to restore windows after help command.
Computes a message and applies the optional argument FUNCTION to it.
If FUNCTION is nil, applies `message' to it, thus printing it."
  (and (not (get-buffer-window standard-output))
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       (let ((first-message
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	      (cond ((special-display-p (buffer-name standard-output))
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		     (setq help-return-method (cons (selected-window) t))
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		     ;; If the help output buffer is a special display buffer,
		     ;; don't say anything about how to get rid of it.
		     ;; First of all, the user will do that with the window
		     ;; manager, not with Emacs.
		     ;; Secondly, the buffer has not been displayed yet,
		     ;; so we don't know whether its frame will be selected.
		     nil)
		    ((not (one-window-p t))
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		     (setq help-return-method
			   (cons (selected-window) 'quit-window))
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		     "Type \\[switch-to-buffer-other-window] RET to restore the other window.")
		    (pop-up-windows
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		     (setq help-return-method (cons (selected-window) t))
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		     "Type \\[delete-other-windows] to remove help window.")
		    (t
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		     (setq help-return-method
			   (list (selected-window) (window-buffer)
				 (window-start) (window-point)))
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		     "Type \\[switch-to-buffer] RET to remove help window."))))
	 (funcall (or function 'message)
		  (concat
		   (if first-message
		       (substitute-command-keys first-message)
		     "")
		   (if first-message "  " "")
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		   ;; If the help buffer will go in a separate frame,
		   ;; it's no use mentioning a command to scroll, so don't.
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		   (if (special-display-p (buffer-name standard-output))
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		       nil
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		     (if (same-window-p (buffer-name standard-output))
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			 ;; Say how to scroll this window.
			 (substitute-command-keys
			  "\\[scroll-up] to scroll the help.")
		       ;; Say how to scroll some other window.
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		       (substitute-command-keys
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			"\\[scroll-other-window] to scroll the help."))))))))
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(defun describe-key (key)
  "Display documentation of the function invoked by KEY.  KEY is a string."
  (interactive "kDescribe key: ")
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  ;; If this key seq ends with a down event, discard the
  ;; following click or drag event.  Otherwise that would
  ;; erase the message.
  (let ((type (aref key (1- (length key)))))
    (if (listp type) (setq type (car type)))
    (and (symbolp type)
	 (memq 'down (event-modifiers type))
	 (read-event)))
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  (save-excursion
    (let ((modifiers (event-modifiers (aref key 0)))
	  window position)
      ;; For a mouse button event, go to the button it applies to
      ;; to get the right key bindings.  And go to the right place
      ;; in case the keymap depends on where you clicked.
      (if (or (memq 'click modifiers) (memq 'down modifiers)
	      (memq 'drag modifiers))
	  (setq window (posn-window (event-start (aref key 0)))
		position (posn-point (event-start (aref key 0)))))
      (if (windowp window)
	  (progn
	    (set-buffer (window-buffer window))
	    (goto-char position)))
      (let ((defn (key-binding key)))
	(if (or (null defn) (integerp defn))
	    (message "%s is undefined" (key-description key))
	  (with-output-to-temp-buffer "*Help*"
	    (princ (key-description key))
	    (if (windowp window)
		(princ " at that spot"))
	    (princ " runs the command ")
	    (prin1 defn)
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	    (princ "\n")
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	    (let ((doc (documentation defn)))
	      (if doc
		  (progn (terpri)
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			 (princ doc)
                         (help-setup-xref (cons #'describe-key key) (interactive-p)))
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		(princ "not documented")))
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	    (print-help-return-message)))))))
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(defun describe-mode ()
  "Display documentation of current major mode and minor modes.
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For this to work correctly for a minor mode, the mode's indicator variable
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\(listed in `minor-mode-alist') must also be a function whose documentation
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describes the minor mode."
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  (interactive)
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  (with-output-to-temp-buffer "*Help*"
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    (let ((minor-modes minor-mode-alist)
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	  (first t))
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      (while minor-modes
	(let* ((minor-mode (car (car minor-modes)))
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	       (indicator (car (cdr (car minor-modes)))))
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	  ;; Document a minor mode if it is listed in minor-mode-alist,
	  ;; bound locally in this buffer, non-nil, and has a function
	  ;; definition.
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	  (if (and (symbol-value minor-mode)
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		   (fboundp minor-mode))
	      (let ((pretty-minor-mode minor-mode))
		(if (string-match "-mode$" (symbol-name minor-mode))
		    (setq pretty-minor-mode
			  (capitalize
			   (substring (symbol-name minor-mode)
				      0 (match-beginning 0)))))
		(while (and indicator (symbolp indicator))
		  (setq indicator (symbol-value indicator)))
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		(if first
		    (princ "The minor modes are described first,
followed by the major mode, which is described on the last page.\n\f\n"))
		(setq first nil)
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		(princ (format "%s minor mode (%s):\n"
			       pretty-minor-mode
			       (if indicator
				   (format "indicator%s" indicator)
				 "no indicator")))
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		(princ (documentation minor-mode))
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		(princ "\n\f\n"))))
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	(setq minor-modes (cdr minor-modes))))
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    (princ mode-name)
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    (princ " mode:\n")
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    (princ (documentation major-mode))
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    (help-setup-xref (cons #'help-xref-mode (current-buffer)) (interactive-p))
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    (print-help-return-message)))

;; So keyboard macro definitions are documented correctly
(fset 'defining-kbd-macro (symbol-function 'start-kbd-macro))

(defun describe-distribution ()
  "Display info on how to obtain the latest version of GNU Emacs."
  (interactive)
  (find-file-read-only
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   (expand-file-name "DISTRIB" data-directory)))
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(defun describe-copying ()
  "Display info on how you may redistribute copies of GNU Emacs."
  (interactive)
  (find-file-read-only
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   (expand-file-name "COPYING" data-directory))
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  (goto-char (point-min)))

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(defun describe-project ()
  "Display info on the GNU project."
  (interactive)
  (find-file-read-only
   (expand-file-name "GNU" data-directory))
  (goto-char (point-min)))

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(defun describe-no-warranty ()
  "Display info on all the kinds of warranty Emacs does NOT have."
  (interactive)
  (describe-copying)
  (let (case-fold-search)
    (search-forward "NO WARRANTY")
    (recenter 0)))

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(defun describe-prefix-bindings ()
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  "Describe the bindings of the prefix used to reach this command.
The prefix described consists of all but the last event
of the key sequence that ran this command."
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  (interactive)
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  (let* ((key (this-command-keys)))
    (describe-bindings
     (if (stringp key)
	 (substring key 0 (1- (length key)))
       (let ((prefix (make-vector (1- (length key)) nil))
	     (i 0))
	 (while (< i (length prefix))
	   (aset prefix i (aref key i))
	   (setq i (1+ i)))
	 prefix)))))
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;; Make C-h after a prefix, when not specifically bound, 
;; run describe-prefix-bindings.
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(setq prefix-help-command 'describe-prefix-bindings)

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(defun view-emacs-news (&optional arg)
  "Display info on recent changes to Emacs.
With numeric argument display information on correspondingly older changes."
  (interactive "P")
  (let* ((arg (if arg (prefix-numeric-value arg) 0)))
    (find-file-read-only
     (expand-file-name (concat (make-string arg ?O) "NEWS")
		       data-directory))))
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(defun view-emacs-FAQ ()
  "Display the Emacs Frequently Asked Questions (FAQ) file."
  (interactive)
  (find-file-read-only (expand-file-name "FAQ" data-directory)))

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(defun view-lossage ()
  "Display last 100 input keystrokes."
  (interactive)
  (with-output-to-temp-buffer "*Help*"
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    (princ (mapconcat (function (lambda (key)
				  (if (or (integerp key)
					  (symbolp key)
					  (listp key))
				      (single-key-description key)
				    (prin1-to-string key nil))))
		      (recent-keys)
		      " "))
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    (save-excursion
      (set-buffer standard-output)
      (goto-char (point-min))
      (while (progn (move-to-column 50) (not (eobp)))
	(search-forward " " nil t)
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	(insert "\n")))
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    (setq help-xref-stack nil
          help-xref-stack-item nil)
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    (print-help-return-message)))

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(defalias 'help 'help-for-help)
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(make-help-screen help-for-help
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  "a b c C f F C-f i I k C-k l L m n p s t v w C-c C-d C-n C-p C-w; ? for help:"
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  "You have typed \\[help-command], the help character.  Type a Help option:
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\(Use SPC or DEL to scroll through this text.  Type \\<help-map>\\[help-quit] to exit the Help command.)
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a  command-apropos.  Give a substring, and see a list of commands
	(functions interactively callable) that contain
	that substring.  See also the  apropos  command.
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b  describe-bindings.  Display table of all key bindings.
c  describe-key-briefly.  Type a command key sequence;
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	it prints the function name that sequence runs.
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C  describe-coding-system.  This describes either a specific coding system
        (if you type its name) or the coding systems currently in use
	(if you type just RET).
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f  describe-function.  Type a function name and get documentation of it.
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C-f Info-goto-emacs-command-node.  Type a function name;
	it takes you to the Info node for that command.
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i  info. The  info  documentation reader.
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I  describe-input-method.  Describe a specific input method (if you type
	its name) or the current input method (if you type just RET).
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k  describe-key.  Type a command key sequence;
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	it displays the full documentation.
C-k Info-goto-emacs-key-command-node.  Type a command key sequence;
	it takes you to the Info node for the command bound to that key.
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l  view-lossage.  Shows last 100 characters you typed.
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L  describe-language-environment.  This describes either the a
	specific language environment (if you type its name)
	or the current language environment (if you type just RET).
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m  describe-mode.  Print documentation of current minor modes,
	and the current major mode, including their special commands.
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n  view-emacs-news.  Shows emacs news file.
p  finder-by-keyword. Find packages matching a given topic keyword.
s  describe-syntax.  Display contents of syntax table, plus explanations
t  help-with-tutorial.  Select the Emacs learn-by-doing tutorial.
v  describe-variable.  Type name of a variable;
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	it displays the variable's documentation and value.
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w  where-is.  Type command name; it prints which keystrokes
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	invoke that command.
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F  Display the frequently asked questions file.
h  Display the HELLO file which illustrates various scripts.
C-c Display Emacs copying permission (General Public License).
C-d Display Emacs ordering information.
C-n Display news of recent Emacs changes.
C-p Display information about the GNU project.
C-w Display information on absence of warranty for GNU Emacs."
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  help-map)
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;; Return a function which is called by the list containing point.
;; If that gives no function, return a function whose name is around point.
;; If that doesn't give a function, return nil.
(defun function-called-at-point ()
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  (let ((stab (syntax-table)))
    (set-syntax-table emacs-lisp-mode-syntax-table)
    (unwind-protect
	(or (condition-case ()
		(save-excursion
		  (save-restriction
		    (narrow-to-region (max (point-min) (- (point) 1000)) (point-max))
		    ;; Move up to surrounding paren, then after the open.
		    (backward-up-list 1)
		    (forward-char 1)
		    ;; If there is space here, this is probably something
		    ;; other than a real Lisp function call, so ignore it.
		    (if (looking-at "[ \t]")
			(error "Probably not a Lisp function call"))
		    (let (obj)
		      (setq obj (read (current-buffer)))
		      (and (symbolp obj) (fboundp obj) obj))))
	      (error nil))
	    (condition-case ()
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		(save-excursion
		  (or (not (zerop (skip-syntax-backward "_w")))
		      (eq (char-syntax (following-char)) ?w)
		      (eq (char-syntax (following-char)) ?_)
		      (forward-sexp -1))
		  (skip-chars-forward "'")
		  (let ((obj (read (current-buffer))))
		    (and (symbolp obj) (fboundp obj) obj)))
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	      (error nil)))
      (set-syntax-table stab))))
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(defun describe-function-find-file (function)
  (let ((files load-history)
	file functions)
    (while files
      (if (memq function (cdr (car files)))
	  (setq file (car (car files)) files nil))
      (setq files (cdr files)))
    file))

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(defun describe-function (function)
  "Display the full documentation of FUNCTION (a symbol)."
  (interactive
   (let ((fn (function-called-at-point))
	 (enable-recursive-minibuffers t)	     
	 val)
     (setq val (completing-read (if fn
				    (format "Describe function (default %s): " fn)
				  "Describe function: ")
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				obarray 'fboundp t nil nil (symbol-name fn)))
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     (list (if (equal val "")
	       fn (intern val)))))
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  (if function
      (with-output-to-temp-buffer "*Help*"
	(prin1 function)
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	;; Use " is " instead of a colon so that
	;; it is easier to get out the function name using forward-sexp.
	(princ " is ")
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	(let* ((def (symbol-function function))
	       file-name
	       (beg (if (commandp def) "an interactive " "a ")))
	  (princ (cond ((or (stringp def)
			    (vectorp def))
			"a keyboard macro")
		       ((subrp def)
			(concat beg "built-in function"))
		       ((byte-code-function-p def)
			(concat beg "compiled Lisp function"))
		       ((symbolp def)
			(format "alias for `%s'" def))
		       ((eq (car-safe def) 'lambda)
			(concat beg "Lisp function"))
		       ((eq (car-safe def) 'macro)
			"a Lisp macro")
		       ((eq (car-safe def) 'mocklisp)
			"a mocklisp function")
		       ((eq (car-safe def) 'autoload)
			(setq file-name (nth 1 def))
			(format "%s autoloaded Lisp %s"
				(if (commandp def) "an interactive" "an")
				(if (nth 4 def) "macro" "function")
				))
		       (t "")))
	  (or file-name
	      (setq file-name (describe-function-find-file function)))
	  (if file-name
	      (progn
		(princ " in `")
		;; We used to add .el to the file name,
		;; but that's completely wrong when the user used load-file.
		(princ file-name)
		(princ "'")))
	  (princ ".")
	  (terpri)
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	  (let* ((inner-function (if (and (listp def) 'macro)
				     (cdr def)
				   def))
		 (arglist (cond ((byte-code-function-p inner-function)
				(car (append inner-function nil)))
			       ((eq (car-safe inner-function) 'lambda)
				(nth 1 inner-function))
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			       (t t))))
	    (if (listp arglist)
		(progn
		  (princ (cons function
			       (mapcar (lambda (arg)
					 (if (memq arg '(&optional &rest))
					     arg
					   (intern (upcase (symbol-name arg)))))
				       arglist)))
		  (terpri))))
	  (let ((doc (documentation function)))
	    (if doc
		(progn (terpri)
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		       (princ doc)
                       (help-setup-xref (cons #'describe-function function) (interactive-p)))
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	      (princ "not documented"))))
	(print-help-return-message)
	(save-excursion
	  (set-buffer standard-output)
	  ;; Return the text we displayed.
	  (buffer-string)))
    (message "You didn't specify a function")))

;; We return 0 if we can't find a variable to return.
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(defun variable-at-point ()
  (condition-case ()
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      (let ((stab (syntax-table)))
	(unwind-protect
	    (save-excursion
	      (set-syntax-table emacs-lisp-mode-syntax-table)
	      (or (not (zerop (skip-syntax-backward "_w")))
		  (eq (char-syntax (following-char)) ?w)
		  (eq (char-syntax (following-char)) ?_)
		  (forward-sexp -1))
	      (skip-chars-forward "'")
	      (let ((obj (read (current-buffer))))
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		(or (and (symbolp obj) (boundp obj) obj)
		    0)))
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	  (set-syntax-table stab)))
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    (error 0)))
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(defun describe-variable (variable)
  "Display the full documentation of VARIABLE (a symbol).
Returns the documentation as a string, also."
  (interactive 
   (let ((v (variable-at-point))
	 (enable-recursive-minibuffers t)
	 val)
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     (setq val (completing-read (if (symbolp v)
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				    (format "Describe variable (default %s): " v)
				  "Describe variable: ")
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				obarray 'boundp t nil nil
				(if (symbolp v) (symbol-name v))))
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     (list (if (equal val "")
	       v (intern val)))))
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  (if (symbolp variable)
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      (let (valvoid)
	(with-output-to-temp-buffer "*Help*"
	  (prin1 variable)
	  (if (not (boundp variable))
	      (progn
		(princ " is void")
		(terpri)
		(setq valvoid t))
	    (princ "'s value is ")
	    (terpri)
	    (pp (symbol-value variable))
	    (terpri))
	  (if (local-variable-p variable)
	      (progn
		(princ (format "Local in buffer %s; " (buffer-name)))
		(if (not (default-boundp variable))
		    (princ "globally void")
		  (princ "global value is ")
		  (terpri)
		  (pp (default-value variable)))
		(terpri)))
	  (terpri)
	  (save-current-buffer
	    (set-buffer standard-output)
	    (if (> (count-lines (point-min) (point-max)) 10)
		(progn
		  (goto-char (point-min))
		  (if valvoid
		      (forward-line 1)
		    (forward-sexp 1)
		    (delete-region (point) (progn (end-of-line) (point)))
		    (insert "'s value is shown below.\n\n")
		    (save-excursion
		      (insert "\n\nValue:"))))))
	  (princ "Documentation:")
	  (terpri)
	  (let ((doc (documentation-property variable 'variable-documentation)))
	    (princ (or doc "not documented as a variable.")))
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          (help-setup-xref (cons #'describe-variable variable) (interactive-p))
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	  (print-help-return-message)
	  (save-excursion
	    (set-buffer standard-output)
	    ;; Return the text we displayed.
	    (buffer-string))))
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    (message "You did not specify a variable")))
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(defun describe-bindings (&optional prefix)
  "Show a list of all defined keys, and their definitions.
We put that list in a buffer, and display the buffer.

The optional argument PREFIX, if non-nil, should be a key sequence;
then we display only bindings that start with that prefix."
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  (interactive "P")
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  (setq help-xref-stack nil
        help-xref-stack-item nil)
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  (describe-bindings-internal nil prefix))

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(defun where-is (definition &optional insert)
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  "Print message listing key sequences that invoke specified command.
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Argument is a command definition, usually a symbol with a function definition.
If INSERT (the prefix arg) is non-nil, insert the message in the buffer."
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  (interactive
   (let ((fn (function-called-at-point))
	 (enable-recursive-minibuffers t)	     
	 val)
     (setq val (completing-read (if fn
				    (format "Where is command (default %s): " fn)
				  "Where is command: ")
				obarray 'fboundp t))
     (list (if (equal val "")
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	       fn (intern val))
	   current-prefix-arg)))
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  (let* ((keys (where-is-internal definition overriding-local-map nil nil))
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	 (keys1 (mapconcat 'key-description keys ", "))
	 (standard-output (if insert (current-buffer) t)))
    (if insert
	(if (> (length keys1) 0)
	    (princ (format "%s (%s)" keys1 definition))
	  (princ (format "M-x %s RET" definition)))
      (if (> (length keys1) 0)
	  (princ (format "%s is on %s" definition keys1))
	(princ (format "%s is not on any key" definition)))))
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  nil)

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(defun locate-library (library &optional nosuffix path interactive-call)
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  "Show the precise file name of Emacs library LIBRARY.
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This command searches the directories in `load-path' like `M-x load-library'
to find the file that `M-x load-library RET LIBRARY RET' would load.
Optional second arg NOSUFFIX non-nil means don't add suffixes `.elc' or `.el'
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to the specified name LIBRARY.

If the optional third arg PATH is specified, that list of directories
is used instead of `load-path'."
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  (interactive (list (read-string "Locate library: ")
		     nil nil
		     t))
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  (let (result)
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    (catch 'answer
      (mapcar
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       (lambda (dir)
	 (mapcar
	  (lambda (suf)
	    (let ((try (expand-file-name (concat library suf) dir)))
	      (and (file-readable-p try)
		   (null (file-directory-p try))
		   (progn
		     (setq result try)
		     (throw 'answer try)))))
	  (if nosuffix
	      '("")
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	    '(".elc" ".el" "")
;;; load doesn't handle this yet.
;;;	    (let ((basic '(".elc" ".el" ""))
;;;		  (compressed '(".Z" ".gz" "")))
;;;	      ;; If autocompression mode is on,
;;;	      ;; consider all combinations of library suffixes
;;;	      ;; and compression suffixes.
;;;	      (if (rassq 'jka-compr-handler file-name-handler-alist)
;;;		  (apply 'nconc
;;;			 (mapcar (lambda (compelt)
;;;				   (mapcar (lambda (baselt)
;;;					     (concat baselt compelt))
;;;					   basic))
;;;				 compressed))
;;;		basic))
	    )))
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       (or path load-path)))
    (and interactive-call
	 (if result
	     (message "Library is file %s" result)
	   (message "No library %s in search path" library)))
    result))
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;;; Grokking cross-reference information in doc strings and
;;; hyperlinking it.

;; This may have some scope for extension and the same or something
;; similar should be done for widget doc strings, which currently use
;; another mechanism.

(defcustom help-highlight-p t
  "*If non-nil, `help-make-xrefs' highlight cross-references.
Under a window system it highlights them with face defined by
`help-highlight-face'.  On a character terminal highlighted
references look like cross-references in info mode."
 :group 'help
 :version "20.3"
 :type 'boolean)

(defcustom help-highlight-face 'underline
  "Face used by `help-make-xrefs' to highlight cross-references.
Must be previously-defined."
  :group 'help
  :version "20.3"
  :type 'symbol)

(defvar help-back-label "[back]"
  "Label to use by `help-make-xrefs' for the go-back reference.")

(defvar help-xref-symbol-regexp
  (concat "\\(\\<\\(\\(variable\\|option\\)\\|"
          "\\(function\\|command\\)\\|"
          "\\(symbol\\)\\)\\s-+\\)?"
          ;; Note starting with word-syntax character:
          "`\\(\\sw\\(\\sw\\|\\s_\\)+\\)'")
  "Regexp matching doc string references to symbols.

The words preceding the quoted symbol can be used in doc strings to
distinguish references to variables, functions and symbols.")

(defvar help-xref-info-regexp
  "\\<info\\s-+node\\s-`\\([^']+\\)'"
  "Regexp matching doc string references to an Info node.")

(defun help-setup-xref (item interactive-p)
  "Invoked from commands using the \"*Help*\" buffer to install some xref info.

ITEM is a (function . args) pair appropriate for recreating the help
buffer after following a reference.  INTERACTIVE-P is non-nil if the
calling command was invoked interactively.  In this case the stack of
items for help buffer \"back\" buttons is cleared."
  (if interactive-p
      (setq help-xref-stack nil))
  (setq help-xref-stack-item item))

(defun help-make-xrefs (&optional buffer)
  "Parse and hyperlink documentation cross-references in the given BUFFER.

Find cross-reference information in a buffer and, if
`help-highlight-p' is non-nil, highlight it with face defined by
`help-highlight-face'; activate such cross references for selection
with `help-follow'.  Cross-references have the canonical form `...'
and the type of reference may be disambiguated by the preceding
word(s) used in `help-xref-symbol-regexp'.

A special reference `back' is made to return back through a stack of
help buffers.  Variable `help-back-label' specifies the text for
that."
  (interactive "b")
  (save-excursion
    (set-buffer (or buffer (current-buffer)))
    (goto-char (point-min))
    ;; Skip the header-type info, though it might be useful to parse
    ;; it at some stage (e.g. "function in `library'").
    (forward-paragraph)
    (let ((old-modified (buffer-modified-p)))
      (let ((stab (syntax-table))
            (case-fold-search t)
            (inhibit-read-only t))
        (set-syntax-table emacs-lisp-mode-syntax-table)
        ;; The following should probably be abstracted out.
        (unwind-protect
            (progn
              ;; Quoted symbols
              (save-excursion
                (while (re-search-forward help-xref-symbol-regexp nil t)
                  (let* ((data (match-string 6))
                         (sym (intern-soft data)))
                    (if sym
                        (cond
                         ((match-string 3) ; `variable' &c
                          (and (boundp sym) ; `variable' doesn't ensure
                                        ; it's actually bound
                               (help-xref-button 6 #'describe-variable sym)))
                         ((match-string 4) ; `function' &c
                          (and (fboundp sym) ; similarly
                               (help-xref-button 6 #'describe-function sym)))
                         ((match-string 5)) ; nothing for symbol
                         ((and (boundp sym) (fboundp sym))
                          ;; We can't intuit whether to use the
                          ;; variable or function doc -- supply both.
                          (help-xref-button 6 #'help-xref-interned sym))
                         ((boundp sym)
                          (help-xref-button 6 #'describe-variable sym))
                         ((fboundp sym)
                          (help-xref-button 6 #'describe-function sym)))))))
              ;; Info references
              (save-excursion
                (while (re-search-forward help-xref-info-regexp nil t)
                  (help-xref-button 1 #'Info-goto-node (list (match-data 1)))))
              ;; An obvious case of a key substitution:
              (save-excursion              
                (while (re-search-forward 
                        "\\<M-x\\s-+\\(\\sw\\(\\sw\\|\\s_\\)+\\)" nil t)
                  (let ((sym (intern-soft (match-string 1))))
                    (if (fboundp sym)
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                        (help-xref-button 1 #'describe-function sym)))))
              ;; Look for commands in whole keymap substitutions:
              (save-excursion
                ;; Find a header and the column at which the command
                ;; name will be found.
                (while (re-search-forward "^key +binding\n\\(-+ +\\)-+\n\n" 
                                          nil t)
                  (let ((col (- (match-end 1) (match-beginning 1))))
                    (while
                        ;; Ignore single blank lines in table, but not
                        ;; double ones, which should terminate it.
                        (and (looking-at "^\n?[^\n]")
                             (progn
                               (if (and (> (move-to-column col) 0)
                                        (looking-at "\\(\\sw\\|\\s_\\)+$"))
                                   ;; 
                                   (let ((sym (intern-soft (match-string 0))))
                                     (if (fboundp sym)
                                         (help-xref-button 
                                          0 #'describe-function sym))))
                               t)
                             (zerop (forward-line))
                             (move-to-column 0)))))))
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          (set-syntax-table stab))
        ;; Make a back-reference in this buffer if appropriate.
        (when help-xref-stack
          (goto-char (point-max))
          (save-excursion
            (insert "\n\n" help-back-label))
          ;; Just to provide the match data:
          (looking-at (concat "\n\n\\(" (regexp-quote help-back-label) "\\)"))
          (help-xref-button 1 #'help-xref-go-back nil)))
      ;; View mode steals RET from us.
      (set (make-local-variable 'minor-mode-overriding-map-alist)
           (list (cons 'view-mode
                       (let ((map (make-sparse-keymap)))
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                         (set-keymap-parent map view-mode-map)
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                         (define-key map "\r" 'help-follow)
                         map))))
      (set-buffer-modified-p old-modified))))

(defun help-xref-button (match-number function data)
  "Make a hyperlink for cross-reference text previously matched.

MATCH-NUMBER is the subexpression of interest in the last matched
regexp.  FUNCTION is a function to invoke when the button is
activated, applied to DATA.  DATA may be a single value or a list.
See `help-make-xrefs'."
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  (add-text-properties (match-beginning match-number)
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                     (match-end match-number)
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                       (list 'mouse-face 'highlight  
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                     'help-xref (cons function
                                      (if (listp data)
                                          data
                                        (list data)))))
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  (if help-highlight-p
      (put-text-property (match-beginning match-number)
                         (match-end match-number)
                         'face help-highlight-face)))
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;; Additional functions for (re-)creating types of help buffers.
(defun help-xref-interned (symbol)
  "Follow a hyperlink which appeared to be an arbitrary interned SYMBOL.

Both variable and function documentation are extracted into a single
help buffer."
  (let ((fdoc (describe-function symbol)))
    (describe-variable symbol)
    ;; We now have a help buffer on the variable.  Insert the function
    ;; text after it.
    (goto-char (point-max))
    (insert "\n\n" fdoc))
  (goto-char (point-min))
  (help-setup-xref (cons #'help-xref-interned symbol) nil))

(defun help-xref-mode (buffer)
  "Do a `describe-mode' for the specified BUFFER."
  (save-excursion
    (set-buffer buffer)
    (describe-mode)))

;;; Navigation/hyperlinking with xrefs

(defun help-follow-mouse (click)
  "Follow the cross-reference that you click on."
  (interactive "e")
  (save-excursion
    (let* ((start (event-start click))
	   (window (car start))
	   (pos (car (cdr start))))
      (set-buffer (window-buffer window))
      (help-follow pos))))

(defun help-xref-go-back ()
  "Go back to the previous help buffer using info on `help-xref-stack'."
  (interactive)
  (when help-xref-stack
    (setq help-xref-stack (cdr help-xref-stack)) ; due to help-follow
    (let* ((item (car help-xref-stack))
           (method (car item))
           (args (cdr item)))
      (setq help-xref-stack (cdr help-xref-stack))
      (if (listp args)
          (apply method args)
        (funcall method args)))))

(defun help-go-back ()
  (interactive)
  (help-follow (1- (point-max))))

(defun help-follow (&optional pos)
  "Follow cross-reference at POS, defaulting to point.

For the cross-reference format, see `help-make-xrefs'."
  (interactive "d")
  (let* ((help-data (get-text-property pos 'help-xref))
         (method (car help-data))
         (args (cdr help-data)))
    (setq help-xref-stack (cons help-xref-stack-item help-xref-stack))
    (setq help-xref-stack-item nil)
    (when help-data
      ;; There is a reference at point.  Follow it.
      (apply method args))))

;; For tabbing through buffer.
(defun help-next-ref ()
  "Find the next help cross-reference in the buffer."
  (interactive)
  (let (pos)
    (while (not pos) 
      (if (get-text-property (point) 'help-xref) ; move off reference
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	   (goto-char (or (next-single-property-change (point) 'help-xref)
                          (point))))
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      (cond ((setq pos (next-single-property-change (point) 'help-xref))
	     (if pos (goto-char pos)))
	    ((bobp)
	     (message "No cross references in the buffer.")
	     (setq pos t))
	    (t				; be circular
	     (goto-char (point-min)))))))

(defun help-previous-ref ()
  "Find the previous help cross-reference in the buffer."
  (interactive)
  (let (pos)
    (while (not pos) 
      (if (get-text-property (point) 'help-xref) ; move off reference
	  (goto-char (or (previous-single-property-change (point) 'help-xref)
                         (point))))
      (cond ((setq pos (previous-single-property-change (point) 'help-xref))
	     (if pos (goto-char pos)))
	    ((bobp)
	     (message "No cross references in the buffer.")
	     (setq pos t))
	    (t				; be circular
	     (goto-char (point-max)))))))

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;;; help.el ends here