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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2000, 2001
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@c   Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
@setfilename ../info/text
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@node Text, Non-ASCII Characters, Markers, Top
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@chapter Text
@cindex text

  This chapter describes the functions that deal with the text in a
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buffer.  Most examine, insert, or delete text in the current buffer,
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often operating at point or on text adjacent to point.  Many are
interactive.  All the functions that change the text provide for undoing
the changes (@pxref{Undo}).
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  Many text-related functions operate on a region of text defined by two
buffer positions passed in arguments named @var{start} and @var{end}.
These arguments should be either markers (@pxref{Markers}) or numeric
character positions (@pxref{Positions}).  The order of these arguments
does not matter; it is all right for @var{start} to be the end of the
region and @var{end} the beginning.  For example, @code{(delete-region 1
10)} and @code{(delete-region 10 1)} are equivalent.  An
@code{args-out-of-range} error is signaled if either @var{start} or
@var{end} is outside the accessible portion of the buffer.  In an
interactive call, point and the mark are used for these arguments.

@cindex buffer contents
  Throughout this chapter, ``text'' refers to the characters in the
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buffer, together with their properties (when relevant).  Keep in mind
that point is always between two characters, and the cursor appears on
the character after point.
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@menu
* Near Point::       Examining text in the vicinity of point.
* Buffer Contents::  Examining text in a general fashion.
* Comparing Text::   Comparing substrings of buffers.
* Insertion::        Adding new text to a buffer.
* Commands for Insertion::  User-level commands to insert text.
* Deletion::         Removing text from a buffer.
* User-Level Deletion::     User-level commands to delete text.
* The Kill Ring::    Where removed text sometimes is saved for later use.
* Undo::             Undoing changes to the text of a buffer.
* Maintaining Undo:: How to enable and disable undo information.
			How to control how much information is kept.
* Filling::          Functions for explicit filling.
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* Margins::          How to specify margins for filling commands.
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* Adaptive Fill::    Adaptive Fill mode chooses a fill prefix from context.
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* Auto Filling::     How auto-fill mode is implemented to break lines.
* Sorting::          Functions for sorting parts of the buffer.
* Columns::          Computing horizontal positions, and using them.
* Indentation::      Functions to insert or adjust indentation.
* Case Changes::     Case conversion of parts of the buffer.
* Text Properties::  Assigning Lisp property lists to text characters.
* Substitution::     Replacing a given character wherever it appears.
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* Transposition::    Swapping two portions of a buffer.
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* Registers::        How registers are implemented.  Accessing the text or
                       position stored in a register.
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* Base 64::          Conversion to or from base 64 encoding.
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* MD5 Checksum::     Compute the MD5 ``message digest''/``checksum''.
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* Change Hooks::     Supplying functions to be run when text is changed.
@end menu

@node Near Point
@section Examining Text Near Point

  Many functions are provided to look at the characters around point.
Several simple functions are described here.  See also @code{looking-at}
in @ref{Regexp Search}.

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@defun char-after &optional position
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This function returns the character in the current buffer at (i.e.,
immediately after) position @var{position}.  If @var{position} is out of
range for this purpose, either before the beginning of the buffer, or at
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or beyond the end, then the value is @code{nil}.  The default for
@var{position} is point.
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In the following example, assume that the first character in the
buffer is @samp{@@}:

@example
@group
(char-to-string (char-after 1))
     @result{} "@@"
@end group
@end example
@end defun

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@defun char-before &optional position
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This function returns the character in the current buffer immediately
before position @var{position}.  If @var{position} is out of range for
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this purpose, either at or before the beginning of the buffer, or beyond
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the end, then the value is @code{nil}.  The default for
@var{position} is point.
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@end defun

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@defun following-char
This function returns the character following point in the current
buffer.  This is similar to @code{(char-after (point))}.  However, if
point is at the end of the buffer, then @code{following-char} returns 0.

Remember that point is always between characters, and the terminal
cursor normally appears over the character following point.  Therefore,
the character returned by @code{following-char} is the character the
cursor is over.

In this example, point is between the @samp{a} and the @samp{c}.

@example
@group
---------- Buffer: foo ----------
Gentlemen may cry ``Pea@point{}ce! Peace!,''
but there is no peace.
---------- Buffer: foo ----------
@end group

@group
(char-to-string (preceding-char))
     @result{} "a"
(char-to-string (following-char))
     @result{} "c"
@end group
@end example
@end defun

@defun preceding-char
This function returns the character preceding point in the current
buffer.  See above, under @code{following-char}, for an example.  If
point is at the beginning of the buffer, @code{preceding-char} returns
0.
@end defun

@defun bobp
This function returns @code{t} if point is at the beginning of the
buffer.  If narrowing is in effect, this means the beginning of the
accessible portion of the text.  See also @code{point-min} in
@ref{Point}.
@end defun

@defun eobp
This function returns @code{t} if point is at the end of the buffer.
If narrowing is in effect, this means the end of accessible portion of
the text.  See also @code{point-max} in @xref{Point}.
@end defun

@defun bolp
This function returns @code{t} if point is at the beginning of a line.
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@xref{Text Lines}.  The beginning of the buffer (or of its accessible
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portion) always counts as the beginning of a line.
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@end defun

@defun eolp
This function returns @code{t} if point is at the end of a line.  The
end of the buffer (or of its accessible portion) is always considered
the end of a line.
@end defun

@node Buffer Contents
@section Examining Buffer Contents

  This section describes two functions that allow a Lisp program to
convert any portion of the text in the buffer into a string.

@defun buffer-substring start end
This function returns a string containing a copy of the text of the
region defined by positions @var{start} and @var{end} in the current
buffer.  If the arguments are not positions in the accessible portion of
the buffer, @code{buffer-substring} signals an @code{args-out-of-range}
error.

It is not necessary for @var{start} to be less than @var{end}; the
arguments can be given in either order.  But most often the smaller
argument is written first.

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If the text being copied has any text properties, these are copied into
the string along with the characters they belong to.  @xref{Text
Properties}.  However, overlays (@pxref{Overlays}) in the buffer and
their properties are ignored, not copied.

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@example
@group
---------- Buffer: foo ----------
This is the contents of buffer foo

---------- Buffer: foo ----------
@end group

@group
(buffer-substring 1 10)
@result{} "This is t"
@end group
@group
(buffer-substring (point-max) 10)
@result{} "he contents of buffer foo
"
@end group
@end example
@end defun

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@defun buffer-substring-no-properties start end
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This is like @code{buffer-substring}, except that it does not copy text
properties, just the characters themselves.  @xref{Text Properties}.
@end defun

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@defun buffer-string
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This function returns the contents of the entire accessible portion of
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the current buffer as a string.  It is equivalent to

@example
(buffer-substring (point-min) (point-max))
@end example
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@example
@group
---------- Buffer: foo ----------
This is the contents of buffer foo

---------- Buffer: foo ----------

(buffer-string)
     @result{} "This is the contents of buffer foo
"
@end group
@end example
@end defun

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@defun thing-at-point thing
Return the @var{thing} around or next to point, as a string.

The argument @var{thing} is a symbol which specifies a kind of syntactic
entity.  Possibilities include @code{symbol}, @code{list}, @code{sexp},
@code{defun}, @code{filename}, @code{url}, @code{word}, @code{sentence},
@code{whitespace}, @code{line}, @code{page}, and others.

@example
---------- Buffer: foo ----------
Gentlemen may cry ``Pea@point{}ce! Peace!,''
but there is no peace.
---------- Buffer: foo ----------

(thing-at-point 'word)
     @result{} "Peace"
(thing-at-point 'line)
     @result{} "Gentlemen may cry ``Peace! Peace!,''\n"
(thing-at-point 'whitespace)
     @result{} nil
@end example
@end defun

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@node Comparing Text
@section Comparing Text
@cindex comparing buffer text

  This function lets you compare portions of the text in a buffer, without
copying them into strings first.

@defun compare-buffer-substrings buffer1 start1 end1 buffer2 start2 end2
This function lets you compare two substrings of the same buffer or two
different buffers.  The first three arguments specify one substring,
giving a buffer and two positions within the buffer.  The last three
arguments specify the other substring in the same way.  You can use
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@code{nil} for @var{buffer1}, @var{buffer2}, or both to stand for the
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current buffer.

The value is negative if the first substring is less, positive if the
first is greater, and zero if they are equal.  The absolute value of
the result is one plus the index of the first differing characters
within the substrings.

This function ignores case when comparing characters
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if @code{case-fold-search} is non-@code{nil}.  It always ignores
text properties.
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Suppose the current buffer contains the text @samp{foobarbar
haha!rara!}; then in this example the two substrings are @samp{rbar }
and @samp{rara!}.  The value is 2 because the first substring is greater
at the second character.

@example
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(compare-buffer-substrings nil 6 11 nil 16 21)
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     @result{} 2
@end example
@end defun

@node Insertion
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@section Inserting Text
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@cindex insertion of text
@cindex text insertion

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@cindex insertion before point
@cindex before point, insertion
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  @dfn{Insertion} means adding new text to a buffer.  The inserted text
goes at point---between the character before point and the character
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after point.  Some insertion functions leave point before the inserted
text, while other functions leave it after.  We call the former
insertion @dfn{after point} and the latter insertion @dfn{before point}.
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  Insertion relocates markers that point at positions after the
insertion point, so that they stay with the surrounding text
(@pxref{Markers}).  When a marker points at the place of insertion,
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insertion may or may not relocate the marker, depending on the marker's
insertion type (@pxref{Marker Insertion Types}).  Certain special
functions such as @code{insert-before-markers} relocate all such markers
to point after the inserted text, regardless of the markers' insertion
type.
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  Insertion functions signal an error if the current buffer is
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read-only or if they insert within read-only text.
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  These functions copy text characters from strings and buffers along
with their properties.  The inserted characters have exactly the same
properties as the characters they were copied from.  By contrast,
characters specified as separate arguments, not part of a string or
buffer, inherit their text properties from the neighboring text.

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  The insertion functions convert text from unibyte to multibyte in
order to insert in a multibyte buffer, and vice versa---if the text
comes from a string or from a buffer.  However, they do not convert
unibyte character codes 128 through 255 to multibyte characters, not
even if the current buffer is a multibyte buffer.  @xref{Converting
Representations}.

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@defun insert &rest args
This function inserts the strings and/or characters @var{args} into the
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current buffer, at point, moving point forward.  In other words, it
inserts the text before point.  An error is signaled unless all
@var{args} are either strings or characters.  The value is @code{nil}.
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@end defun

@defun insert-before-markers &rest args
This function inserts the strings and/or characters @var{args} into the
current buffer, at point, moving point forward.  An error is signaled
unless all @var{args} are either strings or characters.  The value is
@code{nil}.

This function is unlike the other insertion functions in that it
relocates markers initially pointing at the insertion point, to point
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after the inserted text.  If an overlay begins the insertion point, the
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inserted text falls outside the overlay; if a nonempty overlay ends at
the insertion point, the inserted text falls inside that overlay.
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@end defun

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@defun insert-char character count &optional inherit
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This function inserts @var{count} instances of @var{character} into the
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current buffer before point.  The argument @var{count} should be a
number (@code{nil} means 1), and @var{character} must be a character.
The value is @code{nil}.
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This function does not convert unibyte character codes 128 through 255
to multibyte characters, not even if the current buffer is a multibyte
buffer.  @xref{Converting Representations}.

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If @var{inherit} is non-@code{nil}, then the inserted characters inherit
sticky text properties from the two characters before and after the
insertion point.  @xref{Sticky Properties}.
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@end defun

@defun insert-buffer-substring from-buffer-or-name &optional start end
This function inserts a portion of buffer @var{from-buffer-or-name}
(which must already exist) into the current buffer before point.  The
text inserted is the region from @var{start} and @var{end}.  (These
arguments default to the beginning and end of the accessible portion of
that buffer.)  This function returns @code{nil}.

In this example, the form is executed with buffer @samp{bar} as the
current buffer.  We assume that buffer @samp{bar} is initially empty.

@example
@group
---------- Buffer: foo ----------
We hold these truths to be self-evident, that all
---------- Buffer: foo ----------
@end group

@group
(insert-buffer-substring "foo" 1 20)
     @result{} nil

---------- Buffer: bar ----------
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We hold these truth@point{}
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---------- Buffer: bar ----------
@end group
@end example
@end defun

  @xref{Sticky Properties}, for other insertion functions that inherit
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text properties from the nearby text in addition to inserting it.
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Whitespace inserted by indentation functions also inherits text
properties.
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@node Commands for Insertion
@section User-Level Insertion Commands

  This section describes higher-level commands for inserting text,
commands intended primarily for the user but useful also in Lisp
programs.

@deffn Command insert-buffer from-buffer-or-name
This command inserts the entire contents of @var{from-buffer-or-name}
(which must exist) into the current buffer after point.  It leaves
the mark after the inserted text.  The value is @code{nil}.
@end deffn

@deffn Command self-insert-command count
@cindex character insertion
@cindex self-insertion
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This command inserts the last character typed; it does so @var{count}
times, before point, and returns @code{nil}.  Most printing characters
are bound to this command.  In routine use, @code{self-insert-command}
is the most frequently called function in Emacs, but programs rarely use
it except to install it on a keymap.
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In an interactive call, @var{count} is the numeric prefix argument.

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This command calls @code{auto-fill-function} whenever that is
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non-@code{nil} and the character inserted is in the table
@code{auto-fill-chars} (@pxref{Auto Filling}).
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@c Cross refs reworded to prevent overfull hbox.  --rjc 15mar92
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This command performs abbrev expansion if Abbrev mode is enabled and
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the inserted character does not have word-constituent
syntax. (@xref{Abbrevs}, and @ref{Syntax Class Table}.)

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This is also responsible for calling @code{blink-paren-function} when
the inserted character has close parenthesis syntax (@pxref{Blinking}).
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Do not try substituting your own definition of
@code{self-insert-command} for the standard one.  The editor command
loop handles this function specially.
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@end deffn

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@deffn Command newline &optional number-of-newlines
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This command inserts newlines into the current buffer before point.
If @var{number-of-newlines} is supplied, that many newline characters
are inserted.

@cindex newline and Auto Fill mode
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This function calls @code{auto-fill-function} if the current column
number is greater than the value of @code{fill-column} and
@var{number-of-newlines} is @code{nil}.  Typically what
@code{auto-fill-function} does is insert a newline; thus, the overall
result in this case is to insert two newlines at different places: one
at point, and another earlier in the line.  @code{newline} does not
auto-fill if @var{number-of-newlines} is non-@code{nil}.
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This command indents to the left margin if that is not zero.
@xref{Margins}.

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The value returned is @code{nil}.  In an interactive call, @var{count}
is the numeric prefix argument.
@end deffn

@deffn Command split-line
This command splits the current line, moving the portion of the line
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after point down vertically so that it is on the next line directly
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below where it was before.  Whitespace is inserted as needed at the
beginning of the lower line, using the @code{indent-to} function.
@code{split-line} returns the position of point.

Programs hardly ever use this function.
@end deffn

@defvar overwrite-mode
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This variable controls whether overwrite mode is in effect.  The value
should be @code{overwrite-mode-textual}, @code{overwrite-mode-binary},
or @code{nil}.  @code{overwrite-mode-textual} specifies textual
overwrite mode (treats newlines and tabs specially), and
@code{overwrite-mode-binary} specifies binary overwrite mode (treats
newlines and tabs like any other characters).
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@end defvar

@node Deletion
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@section Deleting Text
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@cindex deletion vs killing
  Deletion means removing part of the text in a buffer, without saving
it in the kill ring (@pxref{The Kill Ring}).  Deleted text can't be
yanked, but can be reinserted using the undo mechanism (@pxref{Undo}).
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Some deletion functions do save text in the kill ring in some special
cases.
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  All of the deletion functions operate on the current buffer, and all
return a value of @code{nil}.

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@deffn Command erase-buffer
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This function deletes the entire text of the current buffer, leaving it
empty.  If the buffer is read-only, it signals a @code{buffer-read-only}
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error; if some of the text in it is read-only, it signals a
@code{text-read-only} error.  Otherwise, it deletes the text without
asking for any confirmation.  It returns @code{nil}.
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Normally, deleting a large amount of text from a buffer inhibits further
auto-saving of that buffer ``because it has shrunk''.  However,
@code{erase-buffer} does not do this, the idea being that the future
text is not really related to the former text, and its size should not
be compared with that of the former text.
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@end deffn
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@deffn Command delete-region start end
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This command deletes the text between positions @var{start} and
@var{end} in the current buffer, and returns @code{nil}.  If point was
inside the deleted region, its value afterward is @var{start}.
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Otherwise, point relocates with the surrounding text, as markers do.
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@end deffn

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@defun delete-and-extract-region start end
@tindex delete-and-extract-region
This function deletes the text between positions @var{start} and
@var{end} in the current buffer, and returns a string containing the
text just deleted.

If point was inside the deleted region, its value afterward is
@var{start}.  Otherwise, point relocates with the surrounding text, as
markers do.
@end defun

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@deffn Command delete-char count &optional killp
This command deletes @var{count} characters directly after point, or
before point if @var{count} is negative.  If @var{killp} is
non-@code{nil}, then it saves the deleted characters in the kill ring.

In an interactive call, @var{count} is the numeric prefix argument, and
@var{killp} is the unprocessed prefix argument.  Therefore, if a prefix
argument is supplied, the text is saved in the kill ring.  If no prefix
argument is supplied, then one character is deleted, but not saved in
the kill ring.

The value returned is always @code{nil}.
@end deffn

@deffn Command delete-backward-char count &optional killp
@cindex delete previous char
This command deletes @var{count} characters directly before point, or
after point if @var{count} is negative.  If @var{killp} is
non-@code{nil}, then it saves the deleted characters in the kill ring.

In an interactive call, @var{count} is the numeric prefix argument, and
@var{killp} is the unprocessed prefix argument.  Therefore, if a prefix
argument is supplied, the text is saved in the kill ring.  If no prefix
argument is supplied, then one character is deleted, but not saved in
the kill ring.

The value returned is always @code{nil}.
@end deffn

@deffn Command backward-delete-char-untabify count &optional killp
@cindex tab deletion
This command deletes @var{count} characters backward, changing tabs
into spaces.  When the next character to be deleted is a tab, it is
first replaced with the proper number of spaces to preserve alignment
and then one of those spaces is deleted instead of the tab.  If
@var{killp} is non-@code{nil}, then the command saves the deleted
characters in the kill ring.

Conversion of tabs to spaces happens only if @var{count} is positive.
If it is negative, exactly @minus{}@var{count} characters after point
are deleted.

In an interactive call, @var{count} is the numeric prefix argument, and
@var{killp} is the unprocessed prefix argument.  Therefore, if a prefix
argument is supplied, the text is saved in the kill ring.  If no prefix
argument is supplied, then one character is deleted, but not saved in
the kill ring.

The value returned is always @code{nil}.
@end deffn

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@defopt backward-delete-char-untabify-method
This option specifies how @code{backward-delete-char-untabify} should
deal with whitespace.  Possible values include @code{untabify}, the
default, meaning convert a tab to many spaces and delete one;
@code{hungry}, meaning delete all the whitespace characters before point
with one command, and @code{nil}, meaning do nothing special for
whitespace characters.
@end defopt

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@node User-Level Deletion
@section User-Level Deletion Commands

  This section describes higher-level commands for deleting text,
commands intended primarily for the user but useful also in Lisp
programs.

@deffn Command delete-horizontal-space
@cindex deleting whitespace
This function deletes all spaces and tabs around point.  It returns
@code{nil}.

In the following examples, we call @code{delete-horizontal-space} four
times, once on each line, with point between the second and third
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characters on the line each time.
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@example
@group
---------- Buffer: foo ----------
I @point{}thought
I @point{}     thought
We@point{} thought
Yo@point{}u thought
---------- Buffer: foo ----------
@end group

@group
(delete-horizontal-space)   ; @r{Four times.}
     @result{} nil

---------- Buffer: foo ----------
Ithought
Ithought
Wethought
You thought
---------- Buffer: foo ----------
@end group
@end example
@end deffn

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@deffn Command delete-indentation &optional join-following-p
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This function joins the line point is on to the previous line, deleting
any whitespace at the join and in some cases replacing it with one
space.  If @var{join-following-p} is non-@code{nil},
@code{delete-indentation} joins this line to the following line
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instead.  The function returns @code{nil}.
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If there is a fill prefix, and the second of the lines being joined
starts with the prefix, then @code{delete-indentation} deletes the
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fill prefix before joining the lines.  @xref{Margins}.
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In the example below, point is located on the line starting
@samp{events}, and it makes no difference if there are trailing spaces
in the preceding line.

@smallexample
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@group
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---------- Buffer: foo ----------
When in the course of human
@point{}    events, it becomes necessary
---------- Buffer: foo ----------
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@end group
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(delete-indentation)
     @result{} nil

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@group
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---------- Buffer: foo ----------
When in the course of human@point{} events, it becomes necessary
---------- Buffer: foo ----------
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@end group
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@end smallexample

After the lines are joined, the function @code{fixup-whitespace} is
responsible for deciding whether to leave a space at the junction.
@end deffn

@defun fixup-whitespace
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This function replaces all the whitespace surrounding point with either
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one space or no space, according to the context.  It returns @code{nil}.

At the beginning or end of a line, the appropriate amount of space is
none.  Before a character with close parenthesis syntax, or after a
character with open parenthesis or expression-prefix syntax, no space is
also appropriate.  Otherwise, one space is appropriate.  @xref{Syntax
Class Table}.

In the example below, @code{fixup-whitespace} is called the first time
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with point before the word @samp{spaces} in the first line.  For the
second invocation, point is directly after the @samp{(}.
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@smallexample
@group
---------- Buffer: foo ----------
This has too many     @point{}spaces
This has too many spaces at the start of (@point{}   this list)
---------- Buffer: foo ----------
@end group

@group
(fixup-whitespace)
     @result{} nil
(fixup-whitespace)
     @result{} nil
@end group

@group
---------- Buffer: foo ----------
This has too many spaces
This has too many spaces at the start of (this list)
---------- Buffer: foo ----------
@end group
@end smallexample
@end defun

@deffn Command just-one-space
@comment !!SourceFile simple.el
This command replaces any spaces and tabs around point with a single
space.  It returns @code{nil}.
@end deffn

@deffn Command delete-blank-lines
This function deletes blank lines surrounding point.  If point is on a
blank line with one or more blank lines before or after it, then all but
one of them are deleted.  If point is on an isolated blank line, then it
is deleted.  If point is on a nonblank line, the command deletes all
blank lines following it.

A blank line is defined as a line containing only tabs and spaces.

@code{delete-blank-lines} returns @code{nil}.
@end deffn

@node The Kill Ring
@section The Kill Ring
@cindex kill ring

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  @dfn{Kill functions} delete text like the deletion functions, but save
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it so that the user can reinsert it by @dfn{yanking}.  Most of these
functions have @samp{kill-} in their name.  By contrast, the functions
whose names start with @samp{delete-} normally do not save text for
yanking (though they can still be undone); these are ``deletion''
functions.

  Most of the kill commands are primarily for interactive use, and are
not described here.  What we do describe are the functions provided for
use in writing such commands.  You can use these functions to write
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commands for killing text.  When you need to delete text for internal
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purposes within a Lisp function, you should normally use deletion
functions, so as not to disturb the kill ring contents.
@xref{Deletion}.

  Killed text is saved for later yanking in the @dfn{kill ring}.  This
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is a list that holds a number of recent kills, not just the last text
kill.  We call this a ``ring'' because yanking treats it as having
elements in a cyclic order.  The list is kept in the variable
@code{kill-ring}, and can be operated on with the usual functions for
lists; there are also specialized functions, described in this section,
that treat it as a ring.
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  Some people think this use of the word ``kill'' is unfortunate, since
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it refers to operations that specifically @emph{do not} destroy the
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entities ``killed''.  This is in sharp contrast to ordinary life, in
which death is permanent and ``killed'' entities do not come back to
life.  Therefore, other metaphors have been proposed.  For example, the
term ``cut ring'' makes sense to people who, in pre-computer days, used
scissors and paste to cut up and rearrange manuscripts.  However, it
would be difficult to change the terminology now.

@menu
* Kill Ring Concepts::     What text looks like in the kill ring.
* Kill Functions::         Functions that kill text.
* Yank Commands::          Commands that access the kill ring.
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* Low-Level Kill Ring::	   Functions and variables for kill ring access.
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* Internals of Kill Ring:: Variables that hold kill-ring data.
@end menu

@node Kill Ring Concepts
@comment  node-name,  next,  previous,  up
@subsection Kill Ring Concepts

  The kill ring records killed text as strings in a list, most recent
first.  A short kill ring, for example, might look like this:

@example
("some text" "a different piece of text" "even older text")
@end example

@noindent
When the list reaches @code{kill-ring-max} entries in length, adding a
new entry automatically deletes the last entry.

  When kill commands are interwoven with other commands, each kill
command makes a new entry in the kill ring.  Multiple kill commands in
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succession build up a single kill-ring entry, which would be yanked as a
unit; the second and subsequent consecutive kill commands add text to
the entry made by the first one.
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  For yanking, one entry in the kill ring is designated the ``front'' of
the ring.  Some yank commands ``rotate'' the ring by designating a
different element as the ``front.''  But this virtual rotation doesn't
change the list itself---the most recent entry always comes first in the
list.

@node Kill Functions
@comment  node-name,  next,  previous,  up
@subsection Functions for Killing

  @code{kill-region} is the usual subroutine for killing text.  Any
command that calls this function is a ``kill command'' (and should
probably have @samp{kill} in its name).  @code{kill-region} puts the
newly killed text in a new element at the beginning of the kill ring or
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adds it to the most recent element.  It determines automatically (using
@code{last-command}) whether the previous command was a kill command,
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and if so appends the killed text to the most recent entry.
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@deffn Command kill-region start end
This function kills the text in the region defined by @var{start} and
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@var{end}.  The text is deleted but saved in the kill ring, along with
its text properties.  The value is always @code{nil}.
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In an interactive call, @var{start} and @var{end} are point and
the mark.

@c Emacs 19 feature
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If the buffer or text is read-only, @code{kill-region} modifies the kill
ring just the same, then signals an error without modifying the buffer.
This is convenient because it lets the user use a series of kill
commands to copy text from a read-only buffer into the kill ring.
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@end deffn

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@defopt kill-read-only-ok
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If this option is non-@code{nil}, @code{kill-region} does not signal an
error if the buffer or text is read-only.  Instead, it simply returns,
updating the kill ring but not changing the buffer.
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@end defopt

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@deffn Command copy-region-as-kill start end
This command saves the region defined by @var{start} and @var{end} on
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the kill ring (including text properties), but does not delete the text
from the buffer.  It returns @code{nil}.  It also indicates the extent
of the text copied by moving the cursor momentarily, or by displaying a
message in the echo area.
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The command does not set @code{this-command} to @code{kill-region}, so a
subsequent kill command does not append to the same kill ring entry.

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Don't call @code{copy-region-as-kill} in Lisp programs unless you aim to
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support Emacs 18.  For newer Emacs versions, it is better to use
@code{kill-new} or @code{kill-append} instead.  @xref{Low-Level Kill
Ring}.
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@end deffn

@node Yank Commands
@comment  node-name,  next,  previous,  up
@subsection Functions for Yanking

  @dfn{Yanking} means reinserting an entry of previously killed text
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from the kill ring.  The text properties are copied too.
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@deffn Command yank &optional arg
@cindex inserting killed text
This command inserts before point the text in the first entry in the
kill ring.  It positions the mark at the beginning of that text, and
point at the end.

If @var{arg} is a list (which occurs interactively when the user
types @kbd{C-u} with no digits), then @code{yank} inserts the text as
described above, but puts point before the yanked text and puts the mark
after it.

If @var{arg} is a number, then @code{yank} inserts the @var{arg}th most
recently killed text---the @var{arg}th element of the kill ring list.

@code{yank} does not alter the contents of the kill ring or rotate it.
It returns @code{nil}.
@end deffn

@deffn Command yank-pop arg
This command replaces the just-yanked entry from the kill ring with a
different entry from the kill ring.

This is allowed only immediately after a @code{yank} or another
@code{yank-pop}.  At such a time, the region contains text that was just
inserted by yanking.  @code{yank-pop} deletes that text and inserts in
its place a different piece of killed text.  It does not add the deleted
text to the kill ring, since it is already in the kill ring somewhere.

If @var{arg} is @code{nil}, then the replacement text is the previous
element of the kill ring.  If @var{arg} is numeric, the replacement is
the @var{arg}th previous kill.  If @var{arg} is negative, a more recent
kill is the replacement.

The sequence of kills in the kill ring wraps around, so that after the
oldest one comes the newest one, and before the newest one goes the
oldest.

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The return value is always @code{nil}.
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@end deffn

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@node Low-Level Kill Ring
@subsection Low-Level Kill Ring
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  These functions and variables provide access to the kill ring at a
lower level, but still convenient for use in Lisp programs, because they
take care of interaction with window system selections
(@pxref{Window System Selections}).
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@defun current-kill n &optional do-not-move
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The function @code{current-kill} rotates the yanking pointer, which
designates the ``front'' of the kill ring, by @var{n} places (from newer
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kills to older ones), and returns the text at that place in the ring.
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If the optional second argument @var{do-not-move} is non-@code{nil},
then @code{current-kill} doesn't alter the yanking pointer; it just
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returns the @var{n}th kill, counting from the current yanking pointer.
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If @var{n} is zero, indicating a request for the latest kill,
@code{current-kill} calls the value of
@code{interprogram-paste-function} (documented below) before consulting
the kill ring.
@end defun

@defun kill-new string
This function puts the text @var{string} into the kill ring as a new
entry at the front of the ring.  It discards the oldest entry if
appropriate.  It also invokes the value of
@code{interprogram-cut-function} (see below).
@end defun

@defun kill-append string before-p
This function appends the text @var{string} to the first entry in the
kill ring.  Normally @var{string} goes at the end of the entry, but if
@var{before-p} is non-@code{nil}, it goes at the beginning.  This
function also invokes the value of @code{interprogram-cut-function} (see
below).
@end defun

@defvar interprogram-paste-function
This variable provides a way of transferring killed text from other
programs, when you are using a window system.  Its value should be
@code{nil} or a function of no arguments.

If the value is a function, @code{current-kill} calls it to get the
``most recent kill''.  If the function returns a non-@code{nil} value,
then that value is used as the ``most recent kill''.  If it returns
@code{nil}, then the first element of @code{kill-ring} is used.

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The normal use of this hook is to get the window system's primary
selection as the most recent kill, even if the selection belongs to
another application.  @xref{Window System Selections}.
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@end defvar

@defvar interprogram-cut-function
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This variable provides a way of communicating killed text to other
programs, when you are using a window system.  Its value should be
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@code{nil} or a function of one argument.

If the value is a function, @code{kill-new} and @code{kill-append} call
it with the new first element of the kill ring as an argument.

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The normal use of this hook is to set the window system's primary
selection from the newly killed text.  @xref{Window System Selections}.
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@end defvar

@node Internals of Kill Ring
@comment  node-name,  next,  previous,  up
@subsection Internals of the Kill Ring

  The variable @code{kill-ring} holds the kill ring contents, in the
form of a list of strings.  The most recent kill is always at the front
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of the list.
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  The @code{kill-ring-yank-pointer} variable points to a link in the
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kill ring list, whose @sc{car} is the text to yank next.  We say it
identifies the ``front'' of the ring.  Moving
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@code{kill-ring-yank-pointer} to a different link is called
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@dfn{rotating the kill ring}.  We call the kill ring a ``ring'' because
the functions that move the yank pointer wrap around from the end of the
list to the beginning, or vice-versa.  Rotation of the kill ring is
virtual; it does not change the value of @code{kill-ring}.
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  Both @code{kill-ring} and @code{kill-ring-yank-pointer} are Lisp
variables whose values are normally lists.  The word ``pointer'' in the
name of the @code{kill-ring-yank-pointer} indicates that the variable's
purpose is to identify one element of the list for use by the next yank
command.

  The value of @code{kill-ring-yank-pointer} is always @code{eq} to one
of the links in the kill ring list.  The element it identifies is the
@sc{car} of that link.  Kill commands, which change the kill ring, also
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set this variable to the value of @code{kill-ring}.  The effect is to
rotate the ring so that the newly killed text is at the front.
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  Here is a diagram that shows the variable @code{kill-ring-yank-pointer}
pointing to the second entry in the kill ring @code{("some text" "a
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different piece of text" "yet older text")}.
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@example
@group
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kill-ring                  ---- kill-ring-yank-pointer
  |                       |
  |                       v
  |     --- ---          --- ---      --- ---
   --> |   |   |------> |   |   |--> |   |   |--> nil
        --- ---          --- ---      --- ---
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         |                |            |
         |                |            |
         |                |             -->"yet older text"
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         |                |
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         |                 --> "a different piece of text"
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         |
          --> "some text"
@end group
@end example

@noindent
This state of affairs might occur after @kbd{C-y} (@code{yank})
immediately followed by @kbd{M-y} (@code{yank-pop}).

@defvar kill-ring
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This variable holds the list of killed text sequences, most recently
killed first.
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@end defvar

@defvar kill-ring-yank-pointer
This variable's value indicates which element of the kill ring is at the
``front'' of the ring for yanking.  More precisely, the value is a tail
of the value of @code{kill-ring}, and its @sc{car} is the kill string
that @kbd{C-y} should yank.
@end defvar

@defopt kill-ring-max
The value of this variable is the maximum length to which the kill
ring can grow, before elements are thrown away at the end.  The default
value for @code{kill-ring-max} is 30.
@end defopt

@node Undo
@comment  node-name,  next,  previous,  up
@section Undo
@cindex redo

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  Most buffers have an @dfn{undo list}, which records all changes made
to the buffer's text so that they can be undone.  (The buffers that
don't have one are usually special-purpose buffers for which Emacs
assumes that undoing is not useful.)  All the primitives that modify the
text in the buffer automatically add elements to the front of the undo
list, which is in the variable @code{buffer-undo-list}.
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@defvar buffer-undo-list
This variable's value is the undo list of the current buffer.
A value of @code{t} disables the recording of undo information.
@end defvar

Here are the kinds of elements an undo list can have:

@table @code
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@item @var{position}
This kind of element records a previous value of point; undoing this
element moves point to @var{position}.  Ordinary cursor motion does not
make any sort of undo record, but deletion operations use these entries
to record where point was before the command.
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@item (@var{beg} . @var{end})
This kind of element indicates how to delete text that was inserted.
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Upon insertion, the text occupied the range @var{beg}--@var{end} in the
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buffer.

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@item (@var{text} . @var{position})
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This kind of element indicates how to reinsert text that was deleted.
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The deleted text itself is the string @var{text}.  The place to
reinsert it is @code{(abs @var{position})}.
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@item (t @var{high} . @var{low})
This kind of element indicates that an unmodified buffer became
modified.  The elements @var{high} and @var{low} are two integers, each
recording 16 bits of the visited file's modification time as of when it
was previously visited or saved.  @code{primitive-undo} uses those
values to determine whether to mark the buffer as unmodified once again;
it does so only if the file's modification time matches those numbers.

@item (nil @var{property} @var{value} @var{beg} . @var{end})
This kind of element records a change in a text property.
Here's how you might undo the change:

@example
(put-text-property @var{beg} @var{end} @var{property} @var{value})
@end example
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@item (@var{marker} . @var{adjustment})
This kind of element records the fact that the marker @var{marker} was
relocated due to deletion of surrounding text, and that it moved
@var{adjustment} character positions.  Undoing this element moves
@var{marker} @minus{} @var{adjustment} characters.

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@item nil
This element is a boundary.  The elements between two boundaries are
called a @dfn{change group}; normally, each change group corresponds to
one keyboard command, and undo commands normally undo an entire group as
a unit.
@end table

@defun undo-boundary
This function places a boundary element in the undo list.  The undo
command stops at such a boundary, and successive undo commands undo
to earlier and earlier boundaries.  This function returns @code{nil}.

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The editor command loop automatically creates an undo boundary before
each key sequence is executed.  Thus, each undo normally undoes the
effects of one command.  Self-inserting input characters are an
exception.  The command loop makes a boundary for the first such
character; the next 19 consecutive self-inserting input characters do
not make boundaries, and then the 20th does, and so on as long as
self-inserting characters continue.

All buffer modifications add a boundary whenever the previous undoable
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change was made in some other buffer.  This is to ensure that
each command makes a boundary in each buffer where it makes changes.
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Calling this function explicitly is useful for splitting the effects of
a command into more than one unit.  For example, @code{query-replace}
calls @code{undo-boundary} after each replacement, so that the user can
undo individual replacements one by one.
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@end defun

@defun primitive-undo count list
This is the basic function for undoing elements of an undo list.
It undoes the first @var{count} elements of @var{list}, returning
the rest of @var{list}.  You could write this function in Lisp,
but it is convenient to have it in C.

@code{primitive-undo} adds elements to the buffer's undo list when it
changes the buffer.  Undo commands avoid confusion by saving the undo
list value at the beginning of a sequence of undo operations.  Then the
undo operations use and update the saved value.  The new elements added
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by undoing are not part of this saved value, so they don't interfere with
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continuing to undo.
@end defun

@node Maintaining Undo
@section Maintaining Undo Lists

  This section describes how to enable and disable undo information for
a given buffer.  It also explains how the undo list is truncated
automatically so it doesn't get too big.

  Recording of undo information in a newly created buffer is normally
enabled to start with; but if the buffer name starts with a space, the
undo recording is initially disabled.  You can explicitly enable or
disable undo recording with the following two functions, or by setting
@code{buffer-undo-list} yourself.

@deffn Command buffer-enable-undo &optional buffer-or-name
This command enables recording undo information for buffer
@var{buffer-or-name}, so that subsequent changes can be undone.  If no
argument is supplied, then the current buffer is used.  This function
does nothing if undo recording is already enabled in the buffer.  It
returns @code{nil}.

In an interactive call, @var{buffer-or-name} is the current buffer.
You cannot specify any other buffer.
@end deffn

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@deffn Command buffer-disable-undo &optional buffer
@deffnx Command buffer-flush-undo &optional buffer
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@cindex disable undo
This function discards the undo list of @var{buffer}, and disables
further recording of undo information.  As a result, it is no longer
possible to undo either previous changes or any subsequent changes.  If
the undo list of @var{buffer} is already disabled, this function
has no effect.

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This function returns @code{nil}.
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The name @code{buffer-flush-undo} is not considered obsolete, but the
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preferred name is @code{buffer-disable-undo}.
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@end deffn
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  As editing continues, undo lists get longer and longer.  To prevent
them from using up all available memory space, garbage collection trims
them back to size limits you can set.  (For this purpose, the ``size''
of an undo list measures the cons cells that make up the list, plus the
strings of deleted text.)  Two variables control the range of acceptable
sizes: @code{undo-limit} and @code{undo-strong-limit}.

@defvar undo-limit
This is the soft limit for the acceptable size of an undo list.  The
change group at which this size is exceeded is the last one kept.
@end defvar

@defvar undo-strong-limit
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This is the upper limit for the acceptable size of an undo list.  The
change group at which this size is exceeded is discarded itself (along
with all older change groups).  There is one exception: the very latest
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change group is never discarded no matter how big it is.
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@end defvar

@node Filling
@comment  node-name,  next,  previous,  up
@section Filling
@cindex filling, explicit

  @dfn{Filling} means adjusting the lengths of lines (by moving the line
breaks) so that they are nearly (but no greater than) a specified
maximum width.  Additionally, lines can be @dfn{justified}, which means
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inserting spaces to make the left and/or right margins line up
precisely.  The width is controlled by the variable @code{fill-column}.
For ease of reading, lines should be no longer than 70 or so columns.
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  You can use Auto Fill mode (@pxref{Auto Filling}) to fill text
automatically as you insert it, but changes to existing text may leave
it improperly filled.  Then you must fill the text explicitly.

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  Most of the commands in this section return values that are not
meaningful.  All the functions that do filling take note of the current
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left margin, current right margin, and current justification style
(@pxref{Margins}).  If the current justification style is
@code{none}, the filling functions don't actually do anything.

  Several of the filling functions have an argument @var{justify}.
If it is non-@code{nil}, that requests some kind of justification.  It
can be @code{left}, @code{right}, @code{full}, or @code{center}, to
request a specific style of justification.  If it is @code{t}, that
means to use the current justification style for this part of the text
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(see @code{current-justification}, below).  Any other value is treated
as @code{full}.
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  When you call the filling functions interactively, using a prefix
argument implies the value @code{full} for @var{justify}.
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@deffn Command fill-paragraph justify
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@cindex filling a paragraph
This command fills the paragraph at or after point.  If
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@var{justify} is non-@code{nil}, each line is justified as well.
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It uses the ordinary paragraph motion commands to find paragraph
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boundaries.  @xref{Paragraphs,,, emacs, The GNU Emacs Manual}.
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@end deffn

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@deffn Command fill-region start end &optional justify nosqueeze to-eop
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This command fills each of the paragraphs in the region from @var{start}
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to @var{end}.  It justifies as well if @var{justify} is
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non-@code{nil}.

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If @var{nosqueeze} is non-@code{nil}, that means to leave whitespace
other than line breaks untouched.  If @var{to-eop} is non-@code{nil},
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that means to keep filling to the end of the paragraph---or the next hard
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newline, if @code{use-hard-newlines} is enabled (see below).

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The variable @code{paragraph-separate} controls how to distinguish
paragraphs.  @xref{Standard Regexps}.
@end deffn

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@deffn Command fill-individual-paragraphs start end &optional justify citation-regexp
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This command fills each paragraph in the region according to its
individual fill prefix.  Thus, if the lines of a paragraph were indented
with spaces, the filled paragraph will remain indented in the same
fashion.

The first two arguments, @var{start} and @var{end}, are the beginning
and end of the region to be filled.  The third and fourth arguments,
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@var{justify} and @var{citation-regexp}, are optional.  If
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@var{justify} is non-@code{nil}, the paragraphs are justified as
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well as filled.  If @var{citation-regexp} is non-@code{nil}, it means the
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function is operating on a mail message and therefore should not fill
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the header lines.  If @var{citation-regexp} is a string, it is used as
a regular expression; if it matches the beginning of a line, that line
is treated as a citation marker.
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Ordinarily, @code{fill-individual-paragraphs} regards each change in
indentation as starting a new paragraph.  If
@code{fill-individual-varying-indent} is non-@code{nil}, then only
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separator lines separate paragraphs.  That mode can handle indented
paragraphs with additional indentation on the first line.
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@end deffn

@defopt fill-individual-varying-indent
This variable alters the action of @code{fill-individual-paragraphs} as
described above.
@end defopt

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@deffn Command fill-region-as-paragraph start end &optional justify nosqueeze squeeze-after
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This command considers a region of text as a single paragraph and fills
it.  If the region was made up of many paragraphs, the blank lines
between paragraphs are removed.  This function justifies as well as
filling when @var{justify} is non-@code{nil}.
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In an interactive call, any prefix argument requests justification.
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If @var{nosqueeze} is non-@code{nil}, that means to leave whitespace
other than line breaks untouched.  If @var{squeeze-after} is
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non-@code{nil}, it specifies a position in the region, and means don't
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canonicalize spaces before that position.

In Adaptive Fill mode, this command calls @code{fill-context-prefix} to
choose a fill prefix by default.  @xref{Adaptive Fill}.
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@end deffn

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@deffn Command justify-current-line &optional how eop nosqueeze
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This command inserts spaces between the words of the current line so
that the line ends exactly at @code{fill-column}.  It returns
@code{nil}.
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The argument @var{how}, if non-@code{nil} specifies explicitly the style
of justification.  It can be @code{left}, @code{right}, @code{full},
@code{center}, or @code{none}.  If it is @code{t}, that means to do
follow specified justification style (see @code{current-justification},
below).  @code{nil} means to do full justification.

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If @var{eop} is non-@code{nil}, that means do left-justification if
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@code{current-justification} specifies full justification.  This is used
for the last line of a paragraph; even if the paragraph as a whole is
fully justified, the last line should not be.

If @var{nosqueeze} is non-@code{nil}, that means do not change interior
whitespace.
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@end deffn

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@defopt default-justification
This variable's value specifies the style of justification to use for
text that doesn't specify a style with a text property.  The possible
values are @code{left}, @code{right}, @code{full}, @code{center}, or
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@code{none}.  The default value is @code{left}.
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@end defopt

@defun current-justification
This function returns the proper justification style to use for filling
the text around point.
@end defun

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@defopt sentence-end-double-space
If this variable is non-@code{nil}, a period followed by just one space
does not count as the end of a sentence, and the filling functions
avoid breaking the line at such a place.
@end defopt

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@defvar fill-paragraph-function
This variable provides a way for major modes to override the filling of
paragraphs.  If the value is non-@code{nil}, @code{fill-paragraph} calls
this function to do the work.  If the function returns a non-@code{nil}
value, @code{fill-paragraph} assumes the job is done, and immediately
returns that value.

The usual use of this feature is to fill comments in programming
language modes.  If the function needs to fill a paragraph in the usual
way, it can do so as follows:

@example
(let ((fill-paragraph-function nil))
  (fill-paragraph arg))
@end example
@end defvar

@defvar use-hard-newlines
If this variable is non-@code{nil}, the filling functions do not delete
newlines that have the @code{hard} text property.  These ``hard
newlines'' act as paragraph separators.
@end defvar

@node Margins
@section Margins for Filling

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@defopt fill-prefix
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This buffer-local variable specifies a string of text that appears at
the beginning
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of normal text lines and should be disregarded when filling them.  Any
line that fails to start with the fill prefix is considered the start of
a paragraph; so is any line that starts with the fill prefix followed by
additional whitespace.  Lines that start with the fill prefix but no
additional whitespace are ordinary text lines that can be filled
together.  The resulting filled lines also start with the fill prefix.
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The fill prefix follows the left margin whitespace, if any.
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@end defopt

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@defopt fill-column
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This buffer-local variable specifies the maximum width of filled lines.
Its value should be an integer, which is a number of columns.  All the
filling, justification, and centering commands are affected by this
variable, including Auto Fill mode (@pxref{Auto Filling}).
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As a practical matter, if you are writing text for other people to
read, you should set @code{fill-column} to no more than 70.  Otherwise
the line will be too long for people to read comfortably, and this can
make the text seem clumsy.
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@end defopt
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@defvar default-fill-column
The value of this variable is the default value for @code{fill-column} in
buffers that do not override it.  This is the same as
@code{(default-value 'fill-column)}.

The default value for @code{default-fill-column} is 70.
@end defvar

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@deffn Command set-left-margin from to margin
This sets the @code{left-margin} property on the text from @var{from} to
@var{to} to the value @var{margin}.  If Auto Fill mode is enabled, this
command also refills the region to fit the new margin.
@end deffn

@deffn Command set-right-margin from to margin
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This sets the @code{right-margin} property on the text from @var{from}
to @var{to} to the value @var{margin}.  If Auto Fill mode is enabled,
this command also refills the region to fit the new margin.
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@end deffn

@defun current-left-margin
This function returns the proper left margin value to use for filling
the text around point.  The value is the sum of the @code{left-margin}
property of the character at the start of the current line (or zero if
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none), and the value of the variable @code{left-margin}.
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@end defun

@defun current-fill-column
This function returns the proper fill column value to use for filling
the text around point.  The value is the value of the @code{fill-column}
variable, minus the value of the @code{right-margin} property of the
character after point.
@end defun

@deffn Command move-to-left-margin &optional n force
This function moves point to the left margin of the current line.  The
column moved to is determined by calling the function
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@code{current-left-margin}.  If the argument @var{n} is non-@code{nil},
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@code{move-to-left-margin} moves forward @var{n}@minus{}1 lines first.

If @var{force} is non-@code{nil}, that says to fix the line's
indentation if that doesn't match the left margin value.
@end deffn

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@defun delete-to-left-margin &optional from to
This function removes left margin indentation from the text between
@var{from} and @var{to}.  The amount of indentation to delete is
determined by calling @code{current-left-margin}.  In no case does this
function delete non-whitespace.  If @var{from} and @var{to} are omitted,
they default to the whole buffer.
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@end defun

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@defun indent-to-left-margin
This is the default @code{indent-line-function}, used in Fundamental
mode, Text mode, etc.  Its effect is to adjust the indentation at the
beginning of the current line to the value specified by the variable
@code{left-margin}.  This may involve either inserting or deleting
whitespace.
@end defun

@defvar left-margin
This variable specifies the base left margin column.  In Fundamental
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mode, @kbd{C-j} indents to this column.  This variable automatically
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becomes buffer-local when set in any fashion.
@end defvar

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@defvar fill-nobreak-predicate
This variable gives major modes a way to specify not to break a line at
certain places.  Its value should be a function.  This function is
called during filling, with no arguments and with point located at the
place where a break is being considered.  If the function returns
non-@code{nil}, then the line won't be broken there.
@end defvar