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GNU Emacs NEWS -- history of user-visible changes.
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Copyright (C) 2017-2018 Free Software Foundation, Inc.
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See the end of the file for license conditions.
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Please send Emacs bug reports to bug-gnu-emacs@gnu.org.
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If possible, use M-x report-emacs-bug.

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This file is about changes in Emacs version 27.
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See file HISTORY for a list of GNU Emacs versions and release dates.
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See files NEWS.26, NEWS.25, ..., NEWS.18, and NEWS.1-17 for changes
in older Emacs versions.
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You can narrow news to a specific version by calling 'view-emacs-news'
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with a prefix argument or by typing C-u C-h C-n.
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Temporary note:
+++ indicates that all necessary documentation updates are complete.
    (This means all relevant manuals in doc/ AND lisp doc-strings.)
--- means no change in the manuals is needed.
When you add a new item, use the appropriate mark if you are sure it applies,
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* Installation Changes in Emacs 27.1
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** Emacs now uses GMP, the GNU Multiple Precision library.
By default, if 'configure' does not find a suitable libgmp, it
arranges for the included mini-gmp library to be built and used.
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The new 'configure' option --without-libgmp uses mini-gmp even if a
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suitable libgmp is available.
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** The new configure option '--with-json' adds support for JSON using
the Jansson library.  It is on by default; use 'configure
--with-json=no' to build without Jansson support.  The new JSON
functions 'json-serialize', 'json-insert', 'json-parse-string', and
'json-parse-buffer' are typically much faster than their Lisp
counterparts from json.el.

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** The etags program now uses the C library's regular expression matcher
when possible, and a compatible regex substitute otherwise.  This will
let developers maintain Emacs's own regex code without having to also
support other programs.  The new configure option '--without-included-regex'
forces etags to use the C library's regex matcher even if the regex
substitute ordinarily would be used to work around compatibility problems.

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** Emacs has been ported to the -fcheck-pointer-bounds option of GCC.
This causes Emacs to check bounds of some arrays addressed by its
internal pointers, which can be helpful when debugging the Emacs
interpreter or modules that it uses.  If your platform supports it you
can enable it when configuring, e.g., './configure CFLAGS="-g3 -O2
-mmpx -fcheck-pointer-bounds"' on Intel MPX platforms.

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** Emacs now normally uses a C pointer type instead of a C integer
type to implement Lisp_Object, which is the fundamental machine word
type internal to the Emacs Lisp interpreter.  This change aims to
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catch typos and support -fcheck-pointer-bounds.  The 'configure'
option --enable-check-lisp-object-type is therefore no longer as
useful and so is no longer enabled by default in developer builds,
to reduce differences between developer and production builds.

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** Ibuffer

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---
*** All mode filters can now accept a list of symbols.
This means you can now easily filter several major modes, as well
as a single mode.

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---
*** New toggle 'ibuffer-do-toggle-lock', bound to 'L'.

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** Gnus

+++
*** The nnimap backend now has support for IMAP namespaces.
This feature can be enabled by setting the new 'nnimap-use-namespaces'
server variable to non-nil.

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* Startup Changes in Emacs 27.1
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+++
** Emacs can now be configured using an early init file.
The file is called 'early-init.el', in 'user-emacs-directory'.  It is
loaded very early in the startup process: before graphical elements
such as the tool bar are initialized, and before the package manager
is initialized.  The primary purpose is to allow customizing how the
package system is initialized given that initialization now happens
before loading the regular init file (see below).

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We recommend against putting any customizations in this file that
don't need to be set up before initializing installed add-on packages,
because the early init file is read too early into the startup
process, and some important parts of the Emacs session, such as
window-system and other GUI features, are not yet set up, which could
make some customization fail to work.

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** Installed packages are now activated *before* loading the init file.
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This is part of a change intended to eliminate the behavior of
package.el inserting a call to 'package-initialize' into the init
file, which was previously done when Emacs was started.  As a result
of this change, it is no longer necessary to call 'package-initialize'
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in your init file.

However, if your init file changes the values of 'package-load-list' or
'package-user-dir', or sets 'package-enable-at-startup' to nil then it won't
work right without some adjustment:
- you can move that code to the early init file (see above), so those settings
  apply before Emacs tries to activate the packages.
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- you can use the new 'package-quickstart' so activation of packages does not
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  need to pay attention to 'package-load-list' or 'package-user-dir' any more.
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---
** Emacs now notifies systemd when startup finishes or shutdown begins.
Units that are ordered after 'emacs.service' will only be started
after Emacs has finished initialization and is ready for use.
(If your Emacs is installed in a non-standard location and you copied the
emacs.service file to eg ~/.config/systemd/user/, you will need to copy
the new version of the file again.)

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* Changes in Emacs 27.1
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+++
** The function 'read-passwd' uses '*' as default character to hide passwords.

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---
** New variable 'xft-ignore-color-fonts'.
Default t means don't try to load color fonts when using Xft, as they
often cause crashes.  Set it to nil if you really need those fonts.

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---
** The new option 'tooltip-resize-echo-area' avoids truncating tooltip text
on GUI frames when tooltips are displayed in the echo area.  Instead,
it resizes the echo area as needed to accommodate the full tool-tip
text.

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---
** Show modeline tooltips only if the corresponding action applies.
Customize the option 'mode-line-default-help-echo' to restore the old
behavior where the tooltip text is also shown when the corresponding
action does not apply.

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+++
** New hook 'server-after-make-frame-hook'.
This hook is a convenient place to perform initializations in daemon
mode which require GUI features to be available.  One example is
restoration of the previous session using the desktop.el package: put
the call to 'desktop-read' in this hook, if you want the GUI settings
to be restored, or if desktop.el needs to interact with you during
restoration of the session.

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+++
** New function 'logcount' calculates an integer's Hamming weight.

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+++
** New function 'libxml-available-p'.
This function returns non-nil if libxml support is both compiled in
and available at run time.  Lisp programs should use this function to
detect built-in libxml support, instead of testing for that
indirectly, e.g., by checking that functions like
'libxml-parse-html-region' return nil.

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+++
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** 'libxml-parse-xml-region' and 'libxml-parse-html' region take
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a parameter that's called DISCARD-COMMENTS, but it really only
discards the top-level comment.  Therefore this parameter is now
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obsolete, and the new utility function 'xml-remove-comments' can be
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used to remove comments before calling the libxml functions to parse
the data.

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+++
** The Network Security Manager now allows more fine-grained control
of what checks to run via the `network-security-protocol-checks'
variable.

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** TLS connections have their security tightened by default.
Most of the checks for outdated, believed-to-be-weak TLS algorithms
and ciphers are now switched on by default.  By default, the NSM will
flag connections using these weak algorithms and ask users whether to
allow them.  To get the old behavior back (where certificates are
checked for validity, but no warnings about weak cryptography are
issued), you can either set 'network-security-protocol-checks' to nil,
or adjust the elements in that variable to only happen on the 'high'
security level (assuming you use the 'medium' level).
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** New function 'fill-polish-nobreak-p', to be used in 'fill-nobreak-predicate'.
It blocks line breaking after a one-letter word, also in the case when
this word is preceded by a non-space, but non-alphanumeric character.

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+++
** The limit on repetitions in regexps has been raised to 2^16-1.
It was previously limited to 2^15-1.  For example, the following
regular expression was previously invalid, but is now accepted:

   x\{32768\}

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---
** The German prefix and postfix input methods now support Capital sharp S.

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---
** New input methods hawaiian-postfix and hawaiian-prefix.

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** New function 'exec-path'.
This function by default returns the value of the corresponding
variable, but can optionally return the equivalent of 'exec-path'
from a remote host.
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+++
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** The function 'executable-find' supports an optional argument REMOTE.
This triggers to search the program on the remote host as indicated by
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'default-directory'.

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** New variable 'auto-save-no-message'.
When set to t, no message will be shown when auto-saving (default
value: nil).

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* Editing Changes in Emacs 27.1
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** New command 'make-empty-file'.

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---
** New variable 'x-wait-for-event-timeout'.
This controls how long Emacs will wait for updates to the graphical
state to take effect (making a frame visible, for example).

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+++
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** New user option 'electric-quote-replace-double'.
This option controls whether '"' is replaced in 'electric-quote-mode',
in addition to other quote characters.  If non-nil, ASCII double-quote
characters that quote text "like this" are replaced by double
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typographic quotes, “like this”, in text modes, and in comments in
non-text modes.
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---
** New user option 'flyspell-case-fold-duplications'.
This option controls whether Flyspell mode considers consecutive words
to be duplicates if they are not in the same case.  If non-nil, the
default, words are considered to be duplicates even if their letters'
case does not match.

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---
** 'write-abbrev-file' now includes special properties.
'write-abbrev-file' now writes special properties like ':case-fixed'
for abbrevs that have them.

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+++
** 'insert-abbrev-table-description' skips empty tables.
'insert-abbrev-table-description' skips inserting empty tables when
inserting non-readable tables.  By extension, this makes
'write-abbrev-file' skip writing empty tables.

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** The new functions and commands 'text-property-search-forward' and
'text-property-search-backward' have been added.  These provide an
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interface that's more like functions like @code{search-forward}.

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---
** More commands support noncontiguous rectangular regions, namely
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'upcase-dwim', 'downcase-dwim', 'replace-string', 'replace-regexp'.
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+++
** When asked to visit a large file, Emacs now offers visiting it literally.
Previously, Emacs would only ask for confirmation before visiting
large files.  Now it also offers a third alternative: to visit the
file literally, as in 'find-file-literally', which speeds up
navigation and editing of large files.

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---
** add-dir-local-variable now uses dotted pair notation syntax
to write alists of variables to .dir-locals.el.  This is the same
syntax that you can see in the example of a .dir-locals.el file
in (info "(emacs) Directory Variables")

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* Changes in Specialized Modes and Packages in Emacs 27.1
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** project.el
*** New commands project-search and project-query-replace

** Etags
+++
*** 'next-file' is now an obsolete alias of tags-next-file
*** tags-loop-revert-buffers is an obsolete alias of multifile-revert-buffers
*** The tags-loop-continue function along with the tags-loop-operate and
tags-loop-scan variables are now obsolete; use the new multifile-initialize and
multifile-continue functions instead.

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---
** bibtex
*** New commands 'bibtex-next-entry' and 'bibtex-previous-entry'.
In bibtex-mode-map, forward-paragraph and backward-paragraph are
remapped to these, respectively.

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+++
** Dired

*** New command 'dired-create-empty-file'.

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** Change Logs and VC

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*** Recording ChangeLog entries doesn't require an actual file.
If a ChangeLog file doesn't exist, and if the new variable
'add-log-dont-create-changelog-file' is non-nil (which is the
default), commands such as 'C-x 4 a' will add log entries to a
suitable named temporary buffer.  (An existing ChangeLog file will
still be used if it exists.)  Set the variable to nil to get the
previous behavior of always creating a buffer that visits a ChangeLog
file.
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*** New customizable variable 'vc-git-grep-template'.
This new variable allows customizing the default arguments passed to
git-grep when 'vc-git-grep' is used.

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** diff-mode
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*** Hunks are now automatically refined by default.
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To disable it, set the new defcustom 'diff-font-lock-refine' to nil.

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*** File headers can be shortened, mimicking Magit's diff format.
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To enable it, set the new defcustom 'diff-font-lock-prettify to t.

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** Browse-url
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*** The function 'browse-url-emacs' can now visit a URL in selected window.
It now treats the optional 2nd argument to mean that the URL should be
shown in the currently selected window.

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** Comint
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*** 'send-invisible' is now an obsolete alias for `comint-send-invisible'.
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Also, 'shell-strip-ctrl-m' is declared obsolete.

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+++
*** 'C-c .' (comint-insert-previous-argument) no longer interprets '&'.
This feature caused problems when '&&' was present in the previous
command.  Since this command emulates 'M-.' in Bash and zsh, neither
of which treats '&' specially, the feature was removed for
compatibility with these shells.

+++
*** 'comint-insert-previous-argument' can now count arguments from the end.
By default, invoking 'C-c .' with a numeric argument N would copy the
Nth argument, counting from the first one.  But if the new option
'comint-insert-previous-argument-from-end' is non-nil, it will copy
the Nth argument counting from the last one.  Thus 'C-c .' can now
better emulate 'M-.' in both Bash and zsh, since the former counts
from the beginning of the arguments, while the latter counts from the
end.
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** Term

---
*** 'term-read-noecho' is now obsolete, use 'read-passwd' instead.

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** Flymake

+++
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*** The variable 'flymake-diagnostic-types-alist' is obsolete.
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You should instead set properties on known diagnostic symbols, like
':error' and ':warning', as demonstrated in the Flymake manual.

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*** New customizable variable 'flymake-start-on-save-buffer'.
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Control whether Flymake starts checking the buffer on save.

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*** Flymake and backend functions may exchange hints about buffer changes.
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This enables more efficient backends.  See the docstring of
'flymake-diagnostic-functions' or the Flymake manual for details.

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** Package
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*** New 'package-quickstart' feature.
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When 'package-quickstart' is non-nil, package.el precomputes a big autoloads
file so that activation of packages can be done much faster, which can speed up
your startup significantly.
It also causes variables like package-user-dir and package-load-list to be
consulted when 'package-quickstart-refresh' is run rather than at startup so
you don't need to set them in your early init file.

*** New function 'package-activate-all'.

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** Info

---
*** Info can now follow 'file://' protocol URLs.
The 'file://' URLs in Info documents can now be followed by passing
them to the 'browse-url' function, like the other protocols: ftp,
http, and https.  This allows to have references to local HTML files,
for example.

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** Xref

+++
*** New command 'xref-find-definitions-at-mouse'.
This command finds definitions of the identifier at the place of a
mouse click event, and is intended to be bound to a mouse event.

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** Ecomplete
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*** The ecomplete sorting has changed to a decay-based algorithm.
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This can be controlled by the new 'ecomplete-sort-predicate' variable.
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*** The 'ecompleterc' file is now placed in ~/.emacs.d/ecompleterc by default.
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Of course it will still find it if you have it in ~/.ecompleterc

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** Gnus
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*** A prefix argument to 'gnus-summary-limit-to-score' will limit reverse.
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Limit to articles with score at below.

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*** The function 'gnus-score-find-favorite-words' has been renamed
from 'gnus-score-find-favourite-words'.

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---
*** Gmane has been removed as an nnir backend, since Gmane no longer
has a search engine.

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*** Splitting mail on common mailing list headers has been added.
See the concept index in the Gnus manual for the 'match-list' entry.
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*** nil is no longer an allowed value for 'mm-text-html-renderer'.
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*** A new Gnus summary mode command, 'S A'
('gnus-summary-attach-article') can be used to attach the current
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article(s) to a pre-existing Message buffer, or create a new Message
buffer with the article(s) attached.

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** erc

---
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*** 'erc-button-google-url' has been renamed 'erc-button-search-url'
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and its value has been changed to Duck Duck Go.

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** EUDC

*** XEmacs support has been removed.

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** eww/shr

*** When opening external links in eww/shr (typically with the
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'C-u RET' keystroke on a link), the link will be flashed with the new
'shr-selected-link' face to give the user feedback that the command
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has been executed.

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*** New option 'shr-discard-aria-hidden'.
If set, shr will not render tags with attribute 'aria-hidden="true"'.
This attribute is meant to tell screen readers to ignore a tag.

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** Htmlfontify
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*** The functions 'hfy-color', 'hfy-color-vals' and
'hfy-fallback-color-values' and the variables 'hfy-fallback-color-map'
and 'hfy-rgb-txt-color-map' have been renamed from names that used
'colour' instead of 'color'.

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+++
** Enriched mode supports the 'charset' text property.
You can add or modify the 'charset' text properties of text using the
Edit->Text Properties->Special Properties menu, or by invoking the
'facemenu-set-charset' command.  Documents in Enriched mode will be
saved with the charset properties, and those properties will be
restored when the file is visited.

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** Smtpmail
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Authentication mechanisms can be added via external packages, by
defining new cl-defmethod of smtpmail-try-auth-method.

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** Footnote-mode
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*** Support Hebrew-style footnotes
*** Footnote text lines are now aligned.
Can be controlled via the new variable 'footnote-align-to-fn-text'.

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** CSS mode

---
*** A new command 'css-cycle-color-format' for cycling between color
formats (e.g. "black" => "#000000" => "rgb(0, 0, 0)") has been added,
bound to 'C-c C-f'.

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---
*** CSS mode, SCSS mode, and Less CSS mode now have support for Imenu.

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** SGML mode

---
*** 'sgml-quote' now handles double quotes and apostrophes
when escaping text and in addition all numeric entities when
unescaping text.

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** Python mode

---
*** Python mode supports three different font lock decoration levels.
The maximum level is used by default; customize
'font-lock-maximum-decoration' to tone down the decoration.

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** Dired

+++
*** The new user option 'dired-create-destination-dirs' controls whether
'dired-do-copy' and 'dired-rename-file' should create non-existent
directories in the destination.

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** Help

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---
*** Output format of 'C-h l' (view-lossage) has changed.
For convenience, 'view-lossage' now displays the last keystrokes
and commands in the same format as the edit buffer of
'edit-last-kbd-macro'.  This makes it possible to copy the lines from
the buffer generated by 'view-lossage' to the "*Edit Macro*" buffer
created by 'edit-last-kbd-macro', and to save the macro by 'C-c C-c'.
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---
*** The list of help commands produced by 'C-h C-h' ('help-for-help')
can now be searched via 'C-s'.

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** Ibuffer

---
*** New filter ibuffer-filter-by-process; bound to '/E'.

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** Search and Replace

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+++
*** New isearch bindings.
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'C-M-w' in isearch changed from isearch-del-char to the new function
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isearch-yank-symbol-or-char.  isearch-del-char is now bound to
'C-M-d'.
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*** 'search-exit-option' provides new options 'move' and 'shift-move'
to extend the search string by yanking text that ends at the new
position after moving point in the current buffer.  'shift-move'
extends the search string by motion commands while holding down
the shift key.

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---
*** Isearch now remembers the regexp-based search mode for words/symbols
and case-sensitivity together with search strings in the search ring.

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** Debugger

+++
*** The Lisp Debugger is now based on 'backtrace-mode'.
Backtrace mode adds fontification and commands for changing the
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appearance of backtrace frames.  See the node "Backtraces" in the Elisp
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manual for documentation of the new mode and its commands.

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** Edebug

+++
*** The runtime behavior of Edebug's instrumentation can be changed
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using the new variables 'edebug-behavior-alist',
'edebug-after-instrumentation-function' and
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'edebug-new-definition-function'.  Edebug's behavior can be changed
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globally or for individual definitions.

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*** Edebug's backtrace buffer now uses 'backtrace-mode'.
Backtrace mode adds fontification, links and commands for changing the
appearance of backtrace frames.  See the node "Backtraces" in the Elisp
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manual for documentation of the new mode and its commands.

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The binding of 'd' in Edebug's keymap is now 'edebug-pop-to-backtrace'
which replaces 'edebug-backtrace'.  Consequently Edebug's backtrace
windows now behave like those of the Lisp Debugger and of ERT, in that
when they appear they will be the selected window.

The new 'backtrace-goto-source' command, bound to 's', works in
Edebug's backtraces on backtrace frames whose source code has
been instrumented by Edebug.

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** Enhanced xterm support

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*** New variable 'xterm-set-window-title' controls whether Emacs sets
the XTerm window title.  This feature is experimental and is disabled
by default.
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** grep

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+++
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*** rgrep, lgrep and zrgrep now hide part of the command line
that contains a list of ignored directories and files.
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Clicking on the button with ellipsis unhides it.
The abbreviation can be disabled by the new option
'grep-find-abbreviate'.  The new command
'grep-find-toggle-abbreviation' toggles it interactively.
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** ERT

+++
*** New variable 'ert-quiet' allows to make ERT output in batch mode
less verbose by removing non-essential information.

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+++
*** ERT's backtrace buffer now uses 'backtrace-mode'.
Backtrace mode adds fontification and commands for changing the
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appearance of backtrace frames.  See the node "Backtraces" in the Elisp
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manual for documentation of the new mode and its commands.

** Gamegrid

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---
*** Gamegrid now determines its default glyph size based on display
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dimensions, instead of always using 16 pixels.  As a result, Tetris,
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Snake and Pong are more playable on HiDPI displays.

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** Filecache

---
*** Completing filenames in the minibuffer via 'C-TAB' now uses the
styles as configured by the variable 'completion-styles'.

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** New macros 'thunk-let' and 'thunk-let*'.
These macros are analogue to 'let' and 'let*', but create bindings that
are evaluated lazily.

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** next-error

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+++
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*** New customizable variable 'next-error-find-buffer-function'.
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The value should be a function that determines how to find the
next buffer to be used by 'next-error' and 'previous-error'.  The
default is to use the last buffer that navigated to the current
error.

+++
*** New command 'next-error-select-buffer'.
It can be used to set any buffer as the next one to be used by
'next-error' and 'previous-error'.
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** nxml-mode

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---
*** The default value of 'nxml-sexp-element-flag' is now t.
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This means that pressing C-M-SPACE now selects the entire tree by
default, and not just the opening element.

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** Eshell

---
*** Expansion of history event designators is disabled by default.
To restore the old behavior, use

    (add-hook 'eshell-expand-input-functions
              #'eshell-expand-history-references)

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---
*** The function 'eshell-uniquify-list' has been renamed from
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'eshell-uniqify-list'.

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*** The function eshell/kill is now able to handle signal switches.
Previously eshell/kill would fail if provided a kill signal to send to the
process.  It now accepts signals specified either by name or by its number.

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** Shell

---
*** Program name completion inside remote shells works now as expected.

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** Pcomplete
*** The function 'pcomplete-uniquify-list' has been renamed from
'pcomplete-uniqify-list'.

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** Auth-source

---
*** The Secret Service backend supports the :create key now.

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** Tramp

+++
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*** New connection method "nextcloud", which allows to access OwnCloud
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or NextCloud hosted files and directories.

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+++
*** Connection methods "obex" and "synce" are removed, because they
are obsoleted in GVFS.

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+++
*** Validated passwords are saved by auth-source backends which support this.

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+++
*** The user option 'tramp-ignored-file-name-regexp' allows to disable
Tramp for some look-alike remote file names.

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** Register
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---
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*** The return value of method 'register-val-describe' includes the
names of buffers shown by the windows of a window configuration.

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---
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** The options.el library has been removed.
It was obsolete since Emacs 22.1, replaced by customize.

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** The tls.el and starttls.el libraries are now marked obsolete.
Use of built-in libgnutls based functionality (described in the Emacs
GnuTLS manual) is recommended instead.

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** Message

+++
*** Messages can now be systematically encrypted
when the PGP keyring contains a public key for every recipient.  To
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achieve this, add 'message-sign-encrypt-if-all-keys-available' to
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'message-send-hook'.

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---
*** When replying a message that have addresses on the form
'"foo@bar.com" <foo@bar.com>', Message will elide the repeated "name"
from the address field in the response.

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---
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*** The default of 'message-forward-as-mime' has changed from t to nil
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as it has been reported that many recipients can't read forwards that
are formatted as MIME digests.

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+++
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*** 'message-forward-included-headers' has changed its default to
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exclude most headers when forwarding.

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** EasyPG

---
*** 'epa-pinentry-mode' is renamed to 'epg-pinentry-mode'.
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It now applies to epg functions as well as epa functions.

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---
*** The alias functions 'epa--encode-coding-string',
'epa--decode-coding-string', and 'epa--select-safe-coding-system' have
been removed.  Use 'encode-coding-string', 'decode-coding-string', and
'select-safe-coding-system' instead.

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** Rmail

+++
*** New user option 'rmail-output-reset-deleted-flag'.
If this option is non-nil, messages appended to an output file by the
'rmail-output' command have their Deleted flag reset.

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*** The command 'rmail-summary-by-senders' with an empty argument
selects the messages to summarize with a regexp that matches the
sender of the current message.

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** Threads

+++
*** New variable 'main-thread' holds Emacs's main thread.
This is handy in Lisp programs that run on a non-main thread and want
to signal the main thread, e.g., when they encounter an error.

+++
*** 'thread-join' returns the result of the finished thread now.

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+++
*** 'thread-signal' does not propagate errors to the main thread.
Instead, error messages are just printed in the main thread.

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---
*** 'thread-alive-p' is now obsolete, use 'thread-live-p' instead.

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+++
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*** New command 'list-threads' shows Lisp threads.
See the current list of live threads in a tabulated-list buffer which
automatically updates.  In the buffer, you can use 's q' or 's e' to
signal a thread with quit or error respectively, or get a snapshot
backtrace with 'b'.
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---
** thingatpt.el supports a new "thing" called 'uuid'.
A symbol 'uuid' can be passed to thing-at-point and it returns the
UUID at point.


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** Interactive automatic highlighting

+++
*** 'highlight-regexp' can now highlight subexpressions.
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The now command accepts a prefix numeric argument to choose the
subexpression.
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798

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* New Modes and Packages in Emacs 27.1
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** multifile.el lets one setup multifile operations like search&replace

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+++
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** Emacs can now visit files in archives as if they were directories.
This feature uses Tramp and works only on systems which support GVFS,
i.e. GNU/Linux, roughly spoken.  See the chapter "(tramp) Archive file
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names" in the Tramp manual for full documentation of these facilities.

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+++
** New library for writing JSONRPC applications (https://jsonrpc.org)
The 'jsonrpc' library enables writing Emacs Lisp applications that
rely on this protocol.  Since the protocol is designed to be
transport-agnostic, the library provides an API to implement new
transport strategies as well as a separate API to use them.  A
transport implementation for process-based communication, such as is
used by the Language Server Protocol (LSP), is readily available.

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+++
** Backtrace mode improves viewing of Elisp backtraces.
Backtrace mode adds pretty printing, fontification and ellipsis
expansion to backtrace buffers produced by the Lisp debugger, Edebug
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and ERT.  See the node "Backtraces" in the Elisp manual for
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documentation of the new mode and its commands.

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* Incompatible Lisp Changes in Emacs 27.1

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---
** Just loading a theme's file no longer activates the theme's settings.
Loading a theme with 'M-x load-theme' still activates the theme, as it
did before.  However, loading the theme's file with "M-x load-file",
or using 'require' or 'load' in a Lisp program, doesn't actually apply
the theme's settings until you either invoke 'M-x enable-theme' or
type 'M-x load-theme'.  (In a Lisp program, calling 'enable-theme' or
invoking 'load-theme' with NO-ENABLE argument omitted or nil has the
same effect of activating a theme whose file has been loaded.)  The
special case of the 'user' theme is an exception: it is frequently
used for ad-hoc customizations, so the settings of that theme are by
default applied immediately.

The variable 'custom--inhibit-theme-enable' controls this behavior;
its default value changed in Emacs 27.1.
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** The 'repetitions' argument of 'benchmark-run' can now also be a variable.
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** The FILENAME argument to 'file-name-base' is now mandatory and no
longer defaults to 'buffer-file-name'.

---
** The function 'eldoc-message' now accepts a single argument.
Programs that called it with multiple arguments before should pass
them through 'format' first.  Even that is discouraged: for ElDoc
support, you should set 'eldoc-documentation-function' instead of
calling 'eldoc-message' directly.

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** Old-style backquotes now generate an error.
They have been generating warnings for a decade.  To interpret
old-style backquotes as new-style, bind the new variable
'force-new-style-backquotes' to t.
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** Defining a Common Lisp structure using 'cl-defstruct' or
'cl-struct-define' whose name clashes with a builtin type (e.g.,
'integer' or 'hash-table') now signals an error.

** When formatting a floating-point number as an octal or hexadecimal
integer, Emacs now signals an error if the number is too large for the
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implementation to format.
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---
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** Some functions and variables obsolete since Emacs 22 have been removed:
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archive-mouse-extract, assoc-ignore-case, assoc-ignore-representation,
backward-text-line, blink-cursor, bookmark-exit-hooks,
comint-use-prompt-regexp-instead-of-fields, compilation-finish-function,
count-text-lines, cperl-vc-header-alist, custom-face-save-command,
cvs-display-full-path, cvs-fileinfo->full-path, delete-frame-hook,
derived-mode-class, describe-char-after, describe-project,
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desktop-basefilename, desktop-buffer-handlers, desktop-buffer-misc-functions,
desktop-buffer-modes-to-save, desktop-enable, desktop-load-default,
dired-omit-files-p, disabled-command-hook, dungeon-mode-map,
electric-nroff-mode, electric-nroff-newline, electric-perl-terminator,
focus-frame, forward-text-line, generic-define-mswindows-modes,
generic-define-unix-modes, generic-font-lock-defaults, goto-address-at-mouse,
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highlight-changes-colours, ibuffer-elide-long-columns, ibuffer-hooks,
ibuffer-mode-hooks, icalendar-convert-diary-to-ical,
icalendar-extract-ical-from-buffer, imenu-always-use-completion-buffer-p,
ipconfig-program, ipconfig-program-options, isearch-lazy-highlight-cleanup,
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isearch-lazy-highlight-initial-delay, isearch-lazy-highlight-interval,
isearch-lazy-highlight-max-at-a-time, iswitchb-use-fonts,
latin1-char-displayable-p, mouse-wheel-click-button, mouse-wheel-down-button,
mouse-wheel-up-button, new-frame, pascal-outline, process-kill-without-query,
recentf-menu-append-commands-p, rmail-pop-password,
rmail-pop-password-required, savehist-load, set-default-font,
spam-list-of-processors, speedbar-add-ignored-path-regexp,
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speedbar-buffers-line-path, speedbar-ignored-path-expressions,
speedbar-ignored-path-regexp, speedbar-line-path, speedbar-path-line,
timer-set-time-with-usecs, tooltip-gud-display, tooltip-gud-modes,
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tooltip-gud-toggle-dereference, unfocus-frame, unload-hook-features-list,
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update-autoloads-from-directories, vc-comment-ring, vc-comment-ring-index,
vc-comment-search-forward, vc-comment-search-reverse, vc-comment-to-change-log,
vc-diff-switches-list, vc-next-comment, vc-previous-comment, view-todo,
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x-lost-selection-hooks, x-sent-selection-hooks.

---
** Further functions and variables obsolete since Emacs 24 have been removed:
default-directory-alist, dired-default-directory,
dired-default-directory-alist, dired-enable-local-variables,
dired-hack-local-variables, dired-local-variables-file, dired-omit-here-always.
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** The function 'display-buffer-in-major-side-window' no longer exists.
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It has been renamed as internal function 'window--make-major-side-window',
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however applications should instead call 'display-buffer-in-side-window'
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(passing the SIDE and SLOT parameters as elements of ALIST).  This approach
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is backwards-compatible with versions of Emacs in which the old function
exists.  See the node "Displaying Buffers in Side Windows" in the ELisp
manual for more details.
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** garbage collection no longer treats miscellaneous objects specially;
they are now allocated like any other pseudovector.  As a result, the
'garbage-collect' and 'memory-use-count' functions no longer return a
'misc' component, and the 'misc-objects-consed' variable has been
removed.
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923

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* Lisp Changes in Emacs 27.1
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** lookup-key can take a list of keymaps as argument.

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+++
** 'condition-case' now accepts 't' to match any error symbol.

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+++
** New function 'proper-list-p'.
Given a proper list as argument, this predicate returns its length;
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otherwise, it returns nil.  'format-proper-list-p' is now an obsolete
alias for the new function.
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+++
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** Emacs Lisp integers can now be of arbitrary size.
Emacs uses the GNU Multiple Precision (GMP) library to support
integers whose size is too large to support natively.  The integers
supported natively are known as "fixnums", while the larger ones are
"bignums".  The new predicates 'bignump' and 'fixnump' can be used to
distinguish between these two types of integers.

All the arithmetic, comparison, and logical (a.k.a. "bitwise")
operations where bignums make sense now support both fixnums and
bignums.  However, note that unlike fixnums, bignums will not compare
equal with 'eq', you must use 'eql' instead.  (Numerical comparison
with '=' works on both, of course.)
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Since large bignums consume a lot of memory, Emacs limits the size of
the largest bignum a Lisp program is allowed to create.  The
nonnegative value of the new variable 'integer-width' specifies the
maximum number of bits allowed in a bignum.  Emacs signals an integer
overflow error if this limit is exceeded.
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Several primitive functions formerly returned floats or lists of
integers to represent integers that did not fit into fixnums.  These
functions now simply return integers instead.  Affected functions
include functions like encode-char that compute code-points, functions
like file-attributes that compute file sizes and other attributes,
functions like process-id that compute process IDs, and functions like
user-uid and group-gid that compute user and group IDs.

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+++
** 'time-add', 'time-subtract', and 'time-less-p' now accept
infinities and NaNs too, and propagate them or return nil like
floating-point operators do.

+++
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** New function 'time-equal-p' compares time values for equality.
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** define-minor-mode automatically documents the meaning of ARG.
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+++
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** The function 'recenter' now accepts an additional optional argument.
By default, calling 'recenter' will not redraw the frame even if
'recenter-redisplay' is non-nil.  Call 'recenter' with the new second
argument non-nil to force redisplay per 'recenter-redisplay's value.
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+++
** New functions 'major-mode-suspend' and 'major-mode-restore'.
Use them when switching temporarily to another major mode, e.g. for
'hexl-mode', or to switch between 'c-mode' and 'image-mode' in XPM.
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+++
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** New macro 'dolist-with-progress-reporter'.
This works like 'dolist', but reports progress similar to
'dotimes-with-progress-reporter'.
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+++
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** New hook 'after-delete-frame-functions'.
This works like 'delete-frame-functions', but runs after the frame to
be deleted has been made dead and removed from the frame list.
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---
** The function 'provided-mode-derived-p' was extended to support aliases.
The function now returns non-nil when the argument MODE is derived
from any alias of any of MODES.

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+++
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** New frame focus state inspection interface.
The hooks 'focus-in-hook' and 'focus-out-hook' are now obsolete.
Instead, attach to 'after-focus-change-function' using 'add-function'
and inspect the focus state of each frame using 'frame-focus-state'.
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+++
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** Emacs now requests and recognizes focus-change notifications from TTYs.
On terminal emulators that support the feature, Emacs can now support
'focus-in-hook' and 'focus-out-hook' for TTY frames.
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** Window-specific face remapping.
Face specifications (of the kind used in 'face-remapping-alist')
now support filters, allowing faces to vary between different windows
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displaying the same buffer.  See the Info node "Face Remapping" of the
Emacs Lisp Reference manual for more detail.
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+++
** Special handling of buffer-local 'window-size-change-functions'.
A buffer-local value of this hook is now run only if at least one
window showing the buffer has changed its size.

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+++
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** The function assoc-delete-all now takes an optional predicate argument.
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+++
** New function 'string-distance' to calculate the Levenshtein distance
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between two strings.

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** 'print-quoted' now defaults to t, so if you want to see
(quote x) instead of 'x you will have to bind it to nil where applicable.

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+++
** Numbers formatted via %o or %x may now be formatted as signed integers.
This avoids problems in calls like (read (format "#x%x" -1)), and is
more compatible with bignums, a planned feature.  To get this
behavior, set the experimental variable binary-as-unsigned to nil,
and if the new behavior breaks your code please email
32252@debbugs.gnu.org.  Because %o and %x can now format signed
integers, they now support the + and space flags.

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** To avoid confusion caused by "smart quotes", the reader signals an
error when reading Lisp symbols which begin with one of the following
quotation characters: ‘’‛“”‟〞"'.  A symbol beginning with such a
character can be written by escaping the quotation character with a
backslash.  For example:

    (read "smart") => (invalid-read-syntax "strange quote" "")
    (read "\\smart") == (intern "smart")
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+++
** Omitting variables after '&optional' and '&rest' is now allowed.
For example (defun foo (&optional)) is no longer an error.  This is
sometimes convenient when writing macros.  See the ChangeLog entry
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titled "Allow '&rest' or '&optional' without following variable
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(Bug#29165)" for a full listing of which arglists are accepted across
versions.
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** Internal parsing commands now use 'syntax-ppss' and disregard
'open-paren-in-column-0-is-defun-start'.  This affects mostly things like
'forward-comment', 'scan-sexps', and 'forward-sexp' when parsing backward.
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The new variable 'comment-use-syntax-ppss' can be set to nil to recover the old
behavior if needed.

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** The 'server-name' and 'server-socket-dir' variables are set when a
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socket has been passed to Emacs.
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---
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** The 'file-system-info' function is now available on all platforms.
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instead of just Microsoft platforms.  This fixes a 'get-free-disk-space'
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bug on OS X 10.8 and later.
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+++
** 'memory-limit' now returns a better estimate of memory consumption.

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+++
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** New macro 'combine-change-calls' arranges to call the change hooks
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('before-change-functions' and 'after-change-functions') just once
each around a sequence of lisp forms, given a region.  This is
useful when a function makes a possibly large number of repetitive
changes and the change hooks are time consuming.

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---
** The function 'get-free-disk-space' returns now a non-nil value for
remote systems, which support this check.

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+++
** 'eql', 'make-hash-table', etc. now treat NaNs consistently.
Formerly, some of these functions ignored signs and significands of
NaNs.  Now, all these functions treat NaN signs and significands as
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significant.  For example, (eql 0.0e+NaN -0.0e+NaN) now returns nil
because the two NaNs have different signs; formerly it returned t.
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Also, Emacs now reads and prints NaN significands; e.g., if X is a
NaN, (format "%s" X) now returns "0.0e+NaN", "1.0e+NaN", etc.,
depending on X's significand.
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+++
** The function 'make-string' accepts an additional optional argument.
If the optional third argument is non-nil, 'make-string' will produce
a multibyte string even if its second argument is an ASCII character.

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** (format "%d" X) no longer mishandles a floating-point number X that
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does not fit in a machine integer.
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+++
** In the DST slot, encode-time and parse-time-string now return -1
if it is not known whether daylight saving time is in effect.
Formerly they were inconsistent: encode-time returned t in this
situation, whereas parse-time-string returned nil.  Now they
consistently use use nil to mean that DST is not in effect, and use -1
to mean that it is not known whether DST is in effect.

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** New JSON parsing and serialization functions 'json-serialize',
'json-insert', 'json-parse-string', and 'json-parse-buffer'.  These
are implemented in C using the Jansson library.

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** Mailcap

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---
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*** The new function 'mailcap-file-name-to-mime-type' has been added.
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It's a simple convenience function for looking up MIME types based on
file name extensions.

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*** The default way the list of possible external viewers for MIME
types is sorted and chosen has changed.  Earlier, the most specific
viewer was chosen, even if there was a general override in ~/.mailcap.
For instance, if /etc/mailcap has an entry for image/gif, that one
will be chosen even if you have an entry for image/* in your
~/.mailcap file.  But with the new method, entries from ~/.mailcap
overrides all system and Emacs-provided defaults.  To get the old
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method back, set 'mailcap-prefer-mailcap-viewers' to nil.
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** URL

*** The file: handler no longer looks for index.html in directories if
you ask it for a file:///dir URL.  Since this is a low-level library,
such decisions (if they are to be made at all) are left to
higher-level functions.
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** image-mode

*** image-mode started using ImageMagick by default for all images
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some years back.  It now respects 'imagemagick-types-inhibit' as a way
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