@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993 Free Software Foundation, Inc. 
@c Copyright (C) 1996 Ben Wing.
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/extents.info
@node Extents, Specifiers, Abbrevs, top
@chapter Extents
@cindex extent

  An @dfn{extent} is a region of text (a start position and an end
position) that is displayed in a particular face and can have certain
other properties such as being read-only.  Extents can overlap each
other.  XEmacs efficiently handles buffers with large numbers of
extents in them.

@defun extentp object
This returns @code{t} if @var{object} is an extent.
@end defun

@menu
* Intro to Extents::	   Extents are regions over a buffer or string.
* Creating and Modifying Extents::
			   Basic extent functions.
* Extent Endpoints::	   Accessing and setting the bounds of an extent.
* Finding Extents::	   Determining which extents are in an object.
* Mapping Over Extents::   More sophisticated functions for extent scanning.
* Extent Properties::	   Extents have built-in and user-definable properties.
* Detached Extents::	   Extents that are not in a buffer.
* Extent Parents::         Inheriting properties from another extent.
* Duplicable Extents::	   Extents can be marked to be copied into strings.
* Extents and Events::	   Extents can interact with the keyboard and mouse.
* Atomic Extents::	   Treating a block of text as a single entity.
@end menu

@node Intro to Extents
@section Introduction to Extents
@cindex extent priority
@cindex priority of an extent

  An extent is a region of text within a buffer or string that has
certain properties associated with it.  The properties of an extent
primarily affect the way the text contained in the extent is displayed.
Extents can freely overlap each other in a buffer or string.  Extents
are invisible to functions that merely examine the text of a buffer or
string.

  @emph{NOTE}: An alternative way to add properties to a buffer or
string is to use text properties.  @xref{Text Properties}.

  An extent is logically a Lisp object consisting of a start position,
an end position, a buffer or string to which these positions refer, and
a property list.  As text is inserted into a buffer, the start and end
positions of the extent are automatically adjusted as necessary to keep
the extent referring to the same text in the buffer.  If text is
inserted at the boundary of an extent, the extent's @code{start-open}
and @code{end-open} properties control whether the text is included as
part of the extent.  If the text bounded by an extent is deleted, the
extent becomes @dfn{detached}; its start and end positions are no longer
meaningful, but it maintains all its other properties and can later be
reinserted into a buffer. (None of these considerations apply to strings,
because text cannot be inserted into or deleted from a string.)

  Each extent has a face or list of faces associated with it, which
controls the way in which the text bounded by the extent is displayed.
If an extent's face is @code{nil} or its properties are partially
undefined, the corresponding properties from the default face for the
frame is used.  If two or more extents overlap, or if a list of more
than one face is specified for a particular extent, the corresponding
faces are merged to determine the text's displayed properties.  Every
extent has a @dfn{priority} that determines which face takes precedence
if the faces conflict. (If two extents have the same priority, the one
that comes later in the display order takes precedence.  @xref{Extent
Endpoints, display order}.) Higher-numbered priority values correspond
to a higher priority, and priority values can be negative.  Every extent
is created with a priority of 0, but this can be changed with
@code{set-extent-priority}.  Within a single extent with a list of faces,
faces earlier in the list have a higher priority than faces later in
the list.

  Extents can be set to respond specially to key and mouse events within
the extent.  An extent's @code{keymap} property controls the effect of
key and mouse strokes within the extent's text, and the @code{mouse-face}
property controls whether the extent is highlighted when the mouse moves
over it.  @xref{Extents and Events}.

  An extent can optionally have a @dfn{begin-glyph} or @dfn{end-glyph}
(but not both at one time) associated with it.  A begin-glyph or
end-glyph is a pixmap or string that will be displayed either at the
start or end of an extent or in the margin of the line that the start
or end of the extent lies in, depending on the extent's layout policy.
Begin-glyphs and end-glyphs are used to implement annotations, and you
should use the annotation API functions in preference to the 
lower-level extent functions.  For more information, @xref{Annotations}.

  If an extent has its @code{detachable} property set, it will become
@dfn{detached} (i.e. no longer in the buffer) when all its text its
deleted.  Otherwise, it will simply shrink down to zero-length and
sit it the same place in the buffer.  By default, the @code{detachable}
property is set on newly-created extents.  @xref{Detached Extents}.

  If an extent has its @code{duplicable} property set, it will be
remembered when a string is created from text bounded by the extent.
When the string is re-inserted into a buffer, the extent will also
be re-inserted.  This mechanism is used in the kill, yank, and undo
commands.  @xref{Duplicable Extents}.

@node Creating and Modifying Extents
@section Creating and Modifying Extents

@defun make-extent from to &optional object
This function makes an extent for the range [@var{from}, @var{to}) in
@var{object} (a buffer or string).  @var{object} defaults to the current
buffer.  Insertions at point @var{to} will be outside of the extent;
insertions at @var{from} will be inside the extent, causing the extent
to grow (@pxref{Extent Endpoints}).  This is the same way that markers
behave.  The extent is initially detached if both @var{from} and
@var{to} are @code{nil}, and in this case @var{object} defaults to
@code{nil}, meaning the extent is in no buffer or string
(@pxref{Detached Extents}).
@end defun

@defun delete-extent extent
This function removes @var{extent} from its buffer and destroys it.
This does not modify the buffer's text, only its display properties.
The extent cannot be used thereafter.  To remove an extent in such
a way that it can be re-inserted later, use @code{detach-extent}.
@xref{Detached Extents}.
@end defun

@defun extent-object extent
This function returns the buffer or string that @var{extent} is in.  If
the return value is @code{nil}, this means that the extent is detached;
however, a detached extent will not necessarily return a value of
@code{nil}.
@end defun

@defun extent-live-p extent
This function returns @code{nil} if @var{extent} is deleted, and
@code{t} otherwise.
@end defun

@node Extent Endpoints
@section Extent Endpoints
@cindex extent endpoint
@cindex extent start position
@cindex extent end position
@cindex zero-length extent
@cindex display order
@cindex extent order
@cindex order of extents

  Every extent has a start position and an end position, and logically
affects the characters between those positions.  Normally the start and
end positions must both be valid positions in the extent's buffer or
string.  However, both endpoints can be @code{nil}, meaning the extent
is detached.  @xref{Detached Extents}.

  Whether the extent overlaps its endpoints is governed by its
@code{start-open} and @code{end-open} properties.  Insertion of a
character at a closed endpoint will expand the extent to include that
character; insertion at an open endpoint will not.  Similarly, functions
such as @code{extent-at} that scan over all extents overlapping a
particular position will include extents with a closed endpoint at that
position, but not extents with an open endpoint.

  Note that the @code{start-closed} and @code{end-closed} properties are
equivalent to @code{start-open} and @code{end-open} with the opposite
sense.

  Both endpoints can be equal, in which case the extent includes no
characters but still exists in the buffer or string.  Zero-length
extents are used to represent annotations (@pxref{Annotations}) and can
be used as a more powerful form of a marker.  Deletion of all the
characters in an extent may or may not result in a zero-length extent;
this depends on the @code{detachable} property (@pxref{Detached
Extents}).  Insertion at the position of a zero-length extent expands
the extent if both endpoints are closed; goes before the extent if it
has the @code{start-open} property; and goes after the extent if it has
the @code{end-open} property.  Zero-length extents with both the
@code{start-open} and @code{end-open} properties are treated as if their
starting point were closed.  Deletion of a character on a side of a
zero-length extent whose corresponding endpoint is closed causes the
extent to be detached if its @code{detachable} property is set; if the
corresponding endpoint is open, the extent remains in the buffer, moving
as necessary.

  Extents are ordered within a buffer or string by increasing start
position, and then by decreasing end position (this is called the
@dfn{display order}).

@defun extent-start-position extent
This function returns the start position of @var{extent}.
@end defun

@defun extent-end-position extent
This function returns the end position of @var{extent}.
@end defun

@defun extent-length extent
This function returns the length of @var{extent} in characters.  If
the extent is detached, this returns @code{0}.  If the extent is not
detached, this is equivalent to
@example
(- (extent-end-position @var{extent}) (extent-start-position @var{extent}))
@end example
@end defun

@defun set-extent-endpoints extent start end &optional buffer-or-string
This function sets the start and end position of @var{extent} to
@var{start} and @var{end}.  If both are @code{nil}, this is equivalent
to @code{detach-extent}.

@var{buffer-or-string} specifies the new buffer or string that the
extent should be in, and defaults to @var{extent}'s buffer or
string. (If @code{nil}, and @var{extent} is in no buffer and no string,
it defaults to the current buffer.)

See documentation on @code{detach-extent} for a discussion of undo
recording.
@end defun

@node Finding Extents
@section Finding Extents
@cindex extents, locating

  The following functions provide a simple way of determining the
extents in a buffer or string.  A number of more sophisticated
primitives for mapping over the extents in a range of a buffer or string
are also provided (@pxref{Mapping Over Extents}).  When reading through
this section, keep in mind the way that extents are ordered
(@pxref{Extent Endpoints}).

@defun extent-list &optional buffer-or-string from to flags
This function returns a list of the extents in @var{buffer-or-string}.
@var{buffer-or-string} defaults to the current buffer if omitted.
@var{from} and @var{to} can be used to limit the range over which
extents are returned; if omitted, all extents in the buffer or string
are returned.

  More specifically, if a range is specified using @var{from} and
@var{to}, only extents that overlap the range (i.e. begin or end inside
of the range) are included in the list.  @var{from} and @var{to} default
to the beginning and end of @var{buffer-or-string}, respectively.

  @var{flags} controls how end cases are treated.  For a discussion of
this, and exactly what ``overlap'' means, see @code{map-extents}.
@end defun

  Functions that create extents must be prepared for the possibility
that there are other extents in the same area, created by other 
functions.  To deal with this, functions typically mark their own
extents by setting a particular property on them.  The following
function makes it easier to locate those extents.

@defun extent-at pos &optional object property before at-flag
This function finds the ``smallest'' extent (i.e., the last one in the
display order) at (i.e., overlapping) @var{pos} in @var{object} (a
buffer or string) having @var{property} set.  @var{object} defaults to
the current buffer.  @var{property} defaults to @code{nil}, meaning that
any extent will do.  Returns @code{nil} if there is no matching extent
at @var{pos}.  If the fourth argument @var{before} is not @code{nil}, it
must be an extent; any returned extent will precede that extent.  This
feature allows @code{extent-at} to be used by a loop over extents.

@var{at-flag} controls how end cases are handled (i.e. what ``at''
really means), and should be one of:

@table @code
@item nil
@item after
An extent is at @var{pos} if it covers the character after @var{pos}.
This is consistent with the way that text properties work.
@item before
An extent is at @var{pos} if it covers the character before @var{pos}.
@item at
An extent is at @var{pos} if it overlaps or abuts @var{pos}.  This
includes all zero-length extents at @var{pos}.
@end table

  Note that in all cases, the start-openness and end-openness of the
extents considered is ignored.  If you want to pay attention to those
properties, you should use @code{map-extents}, which gives you more
control.
@end defun

  The following low-level functions are provided for explicitly
traversing the extents in a buffer according to the display order.
These functions are mostly intended for debugging -- in normal
operation, you should probably use @code{mapcar-extents} or
@code{map-extents}, or loop using the @var{before} argument to
@code{extent-at}, rather than creating a loop using @code{next-extent}.

@defun next-extent extent
Given an extent @var{extent}, this function returns the next extent in
the buffer or string's display order.  If @var{extent} is a buffer or
string, this returns the first extent in the buffer or string.
@end defun

@defun previous-extent extent
Given an extent @var{extent}, this function returns the previous extent
in the buffer or string's display order.  If @var{extent} is a buffer or
string, this returns the last extent in the buffer or string.
@end defun

@node Mapping Over Extents
@section Mapping Over Extents
@cindex extents, mapping

  The most basic and general function for mapping over extents is called
@code{map-extents}.  You should read through the definition of this
function to familiarize yourself with the concepts and optional
arguments involved.  However, in practice you may find it more
convenient to use the function @code{mapcar-extents} or to create a loop
using the @code{before} argument to @code{extent-at} (@pxref{Finding
Extents}).

@defun map-extents function &optional object from to maparg flags property value
  This function maps @var{function} over the extents which overlap a
region in @var{object}.  @var{object} is normally a buffer or string but
could be an extent (see below).  The region is normally bounded by
[@var{from}, @var{to}) (i.e. the beginning of the region is closed and
the end of the region is open), but this can be changed with the
@var{flags} argument (see below for a complete discussion).

  @var{function} is called with the arguments (extent, @var{maparg}).
The arguments @var{object}, @var{from}, @var{to}, @var{maparg}, and
@var{flags} are all optional and default to the current buffer, the
beginning of @var{object}, the end of @var{object}, @var{nil}, and
@var{nil}, respectively.  @code{map-extents} returns the first
non-@code{nil} result produced by @var{function}, and no more calls to
@var{function} are made after it returns non-@code{nil}.

  If @var{object} is an extent, @var{from} and @var{to} default to the
extent's endpoints, and the mapping omits that extent and its
predecessors.  This feature supports restarting a loop based on
@code{map-extents}.  Note: @var{object} must be attached to a buffer or
string, and the mapping is done over that buffer or string.

  An extent overlaps the region if there is any point in the extent that
is also in the region. (For the purpose of overlap, zero-length extents
and regions are treated as closed on both ends regardless of their
endpoints' specified open/closedness.) Note that the endpoints of an
extent or region are considered to be in that extent or region if and
only if the corresponding end is closed.  For example, the extent [5,7]
overlaps the region [2,5] because 5 is in both the extent and the
region.  However, (5,7] does not overlap [2,5] because 5 is not in the
extent, and neither [5,7] nor (5,7] overlaps the region [2,5) because 5
is not in the region.

  The optional @var{flags} can be a symbol or a list of one or more
symbols, modifying the behavior of @code{map-extents}.  Allowed symbols
are:

@table @code
@item end-closed
The region's end is closed.

@item start-open
The region's start is open.

@item all-extents-closed
Treat all extents as closed on both ends for the purpose of determining
whether they overlap the region, irrespective of their actual open- or
closedness.
@item all-extents-open
Treat all extents as open on both ends.
@item all-extents-closed-open
Treat all extents as start-closed, end-open.
@item all-extents-open-closed
Treat all extents as start-open, end-closed.

@item start-in-region
In addition to the above conditions for extent overlap, the extent's
start position must lie within the specified region.  Note that, for
this condition, open start positions are treated as if 0.5 was added to
the endpoint's value, and open end positions are treated as if 0.5 was
subtracted from the endpoint's value.
@item end-in-region
The extent's end position must lie within the region.
@item start-and-end-in-region
Both the extent's start and end positions must lie within the region.
@item start-or-end-in-region
Either the extent's start or end position must lie within the region.

@item negate-in-region
The condition specified by a @code{*-in-region} flag must @emph{not}
hold for the extent to be considered.
@end table

  At most one of @code{all-extents-closed}, @code{all-extents-open},
@code{all-extents-closed-open}, and @code{all-extents-open-closed} may
be specified.

  At most one of @code{start-in-region}, @code{end-in-region},
@code{start-and-end-in-region}, and @code{start-or-end-in-region} may be
specified.

  If optional arg @var{property} is non-@code{nil}, only extents with
that property set on them will be visited.  If optional arg @var{value}
is non-@code{nil}, only extents whose value for that property is
@code{eq} to @var{value} will be visited.
@end defun

  If you want to map over extents and accumulate a list of results,
the following function may be more convenient than @code{map-extents}.

@defun mapcar-extents function &optional predicate buffer-or-string from to flags property value
This function applies @var{function} to all extents which overlap a
region in @var{buffer-or-string}.  The region is delimited by
@var{from} and @var{to}.  @var{function} is called with one argument,
the extent.  A list of the values returned by @var{function} is
returned.  An optional @var{predicate} may be used to further limit the
extents over which @var{function} is mapped.  The optional arguments
@var{flags}, @var{property}, and @var{value} may also be used to control
the extents passed to @var{predicate} or @var{function}, and have the
same meaning as in @code{map-extents}.
@end defun

@defun map-extent-children function &optional object from to maparg flags property value
This function is similar to @code{map-extents}, but differs in that:

@itemize @bullet
@item
It only visits extents which start in the given region.
@item
After visiting an extent @var{e}, it skips all other extents which start
inside @var{e} but end before @var{e}'s end.
@end itemize

Thus, this function may be used to walk a tree of extents in a buffer:
@example
(defun walk-extents (buffer &optional ignore)
  (map-extent-children 'walk-extents buffer))
@end example
@end defun

@defun extent-in-region-p extent &optional from to flags
This function returns @var{t} if @code{map-extents} would visit
@var{extent} if called with the given arguments.
@end defun

@node Extent Properties
@section Properties of Extents
@cindex extent property
@cindex property of an extent

  Each extent has a property list associating property names with
values.  Some property names have predefined meanings, and can usually
only assume particular values.  Assigning other values to such a
property either cause the value to be converted into a legal value
(e.g., assigning anything but @code{nil} to a Boolean property will
cause the value of @code{t} to be assigned to the property) or will
cause an error.  Property names without predefined meanings can be
assigned any value.  An undefined property is equivalent to a property
with a value of @code{nil}, or with a particular default value in the
case of properties with predefined meanings.  Note that, when an extent
is created, the @code{end-open} and @code{detachable} properties are set
on it.

  If an extent has a parent, all of its properties actually derive
from that parent (or from the root ancestor if the parent in turn
has a parent), and setting a property of the extent actually sets
that property on the parent.  @xref{Extent Parents}.

@defun extent-property extent property
This function returns the value of @var{property} in @var{extent}.  If
@var{property} is undefined, @code{nil} is returned.
@end defun

@defun extent-properties extent
This function returns a list of all of @var{extent}'s properties that do
not have the value of @code{nil} (or the default value, for properties
with predefined meanings).
@end defun

@defun set-extent-property extent property value
This function sets @var{property} to @var{value} in @var{extent}. (If
@var{property} has a predefined meaning, only certain values are
allowed, and some values may be converted to others before being
stored.)
@end defun

The following table lists the properties with predefined meanings, along
with their allowable values.

@table @code
@item detached
(Boolean) Whether the extent is detached.   Setting this is the same
as calling @code{detach-extent}.  @xref{Detached Extents}.

@item destroyed
(Boolean) Whether the extent has been deleted.  Setting this is the same
as calling @code{delete-extent}.

@item priority
(integer) The extent's redisplay priority.  Defaults to 0.  @xref{Intro
to Extents, priority}.  This property can also be set with
@code{set-extent-priority} and accessed with @code{extent-priority}.

@item start-open
(Boolean) Whether the start position of the extent is open, meaning that
characters inserted at that position go outside of the extent.
@xref{Extent Endpoints}.

@item start-closed
(Boolean) Same as @code{start-open} but with the opposite sense.  Setting
this property clears @code{start-open} and vice-versa.

@item end-open
(Boolean) Whether the end position of the extent is open, meaning that
characters inserted at that position go outside of the extent.  This is
@code{t} by default.
@xref{Extent Endpoints}.

@item end-closed
(Boolean) Same as @code{end-open} but with the opposite sense.  Setting
this property clears @code{end-open} and vice-versa.

@item read-only
(Boolean) Whether text within this extent will be unmodifiable.

@item face
(face, face name, list of faces or face names, or @code{nil}) The face
in which to display the extent's text.  This property can also be set
with @code{set-extent-face} and accessed with @code{extent-face}.
Note that if a list of faces is specified, the faces are merged together,
with faces earlier in the list having priority over faces later in the
list.

@item mouse-face
(face, face name, list of faces or face names, or @code{nil}) The face
used to display the extent when the mouse moves over it.  This property
can also be set with @code{set-extent-mouse-face} and accessed with
@code{extent-mouse-face}.  Note that if a list of faces is specified,
the faces are merged together, with faces earlier in the list having
priority over faces later in the list.  @xref{Extents and Events}.

@item pointer
(pointer glyph)  The glyph used as the pointer when the mouse moves over
the extent.  This takes precedence over the @code{text-pointer-glyph}
and @code{nontext-pointer-glyph} variables.  If for any reason this
glyph is an invalid pointer, the standard glyphs will be used as
fallbacks.  @xref{Mouse Pointer}

@item detachable
(Boolean) Whether this extent becomes detached when all of the text it
covers is deleted.  This is @code{t} by default.  @xref{Detached
Extents}.

@item duplicable
(Boolean) Whether this extent should be copied into strings, so that
kill, yank, and undo commands will restore or copy it.  @xref{Duplicable
Extents}.

@item unique
(Boolean) Meaningful only in conjunction with @code{duplicable}.
When this is set, there may be only one instance of
this extent attached at a time.  @xref{Duplicable Extents}.

@item invisible
(Boolean) If @code{t}, text under this extent will not be displayed --
it will look as if the text is not there at all.

@item keymap
(keymap or @code{nil}) This keymap is consulted for mouse clicks on this
extent or keypresses made while @code{point} is within the extent.
@xref{Extents and Events}.

@item copy-function
This is a hook that is run when a duplicable extent is about to be
copied from a buffer to a string (or the kill ring).  @xref{Duplicable
Extents}.

@item paste-function
This is a hook that is run when a duplicable extent is about to be
copied from a string (or the kill ring) into a buffer.  @xref{Duplicable
Extents}.

@item begin-glyph
(glyph or @code{nil}) This extent's begin glyph.
@xref{Annotations}.

@item end-glyph
(glyph or @code{nil}) This extent's end glyph.
@xref{Annotations}.

@item begin-glyph-layout
(@code{text}, @code{whitespace}, @code{inside-margin}, or
@code{outside-margin}) The layout policy for this extent's begin glyph.
Defaults to @code{text}.  @xref{Annotations}.

@item end-glyph-layout
(@code{text}, @code{whitespace}, @code{inside-margin}, or
@code{outside-margin}) The layout policy for this extent's end glyph.
Defaults to @code{text}.  @xref{Annotations}.
@end table

The following convenience functions are provided for accessing
particular properties of an extent.

@defun extent-face extent
This function returns the @code{face} property of @var{extent}.  This
might also return a list of face names.  Do not modify this list
directly!  Instead, use @code{set-extent-face}.

Note that you can use @code{eq} to compare lists of faces as returned
by @code{extent-face}.  In other words, if you set the face of two
different extents to two lists that are @code{equal} but not @code{eq},
then the return value of @code{extent-face} on the two extents will
return the identical list.
@end defun

@defun extent-mouse-face extent
This function returns the @code{mouse-face} property of @var{extent}.
This might also return a list of face names.  Do not modify this list
directly!  Instead, use @code{set-extent-mouse-face}.

Note that you can use @code{eq} to compare lists of faces as returned
by @code{extent-mouse-face}, just like for @code{extent-face}.
@end defun

@defun extent-priority extent
This function returns the @code{priority} property of @var{extent}.
@end defun

@defun extent-begin-glyph-layout extent
This function returns the @code{begin-glyph-layout} property of
@var{extent}, i.e. the layout policy associated with the @var{extent}'s
begin glyph.
@end defun

@defun extent-end-glyph-layout extent
This function returns the @code{end-glyph-layout} property of
@var{extent}, i.e. the layout policy associated with the @var{extent}'s
end glyph.
@end defun

@defun extent-begin-glyph extent
This function returns the @code{begin-glyph} property of @var{extent},
i.e. the glyph object displayed at the beginning of @var{extent}.  If
there is none, @code{nil} is returned.
@end defun

@defun extent-end-glyph extent
This function returns the @code{end-glyph} property of @var{extent},
i.e. the glyph object displayed at the end of @var{extent}.  If
there is none, @code{nil} is returned.
@end defun

The following convenience functions are provided for setting particular
properties of an extent.

@defun set-extent-priority extent pri
This function sets the @code{priority} property of @var{extent} to
@var{pri}.
@end defun

@defun set-extent-face extent face
This function sets the @code{face} property of @var{extent} to
@var{face}.
@end defun

@defun set-extent-mouse-face extent face
This function sets the @code{mouse-face} property of @var{extent} to
@var{face}.
@end defun

@defun set-extent-keymap extent keymap
This function sets the @code{keymap} property of @var{extent} to
@var{keymap}.
@end defun

@defun set-extent-begin-glyph-layout extent layout
This function sets the @code{begin-glyph-layout} property of
@var{extent} to @var{layout}.
@end defun

@defun set-extent-end-glyph-layout extent layout
This function sets the @code{end-glyph-layout} property of
@var{extent} to @var{layout}.
@end defun

@defun set-extent-begin-glyph extent begin-glyph &optional layout
This function sets the @code{begin-glyph} and @code{glyph-layout}
properties of @var{extent} to @var{begin-glyph} and @var{layout},
respectively. (@var{layout} defaults to @code{text} if not specified.)
@end defun

@defun set-extent-end-glyph extent end-glyph &optional layout
This function sets the @code{end-glyph} and @code{glyph-layout}
properties of @var{extent} to @var{end-glyph} and @var{layout},
respectively. (@var{layout} defaults to @code{text} if not specified.)
@end defun

@node Detached Extents
@section Detached Extents
@cindex detached extent

A detached extent is an extent that is not attached to a buffer or
string but can be re-inserted.  Detached extents have a start position
and end position of @code{nil}.  Extents can be explicitly detached
using @code{detach-extent}.  An extent is also detached when all of its
characters are all killed by a deletion, if its @code{detachable}
property is set; if this property is not set, the extent becomes a
zero-length extent. (Zero-length extents with the @code{detachable}
property set behave specially.  @xref{Extent Endpoints, zero-length
extents}.)

@defun detach-extent extent
This function detaches @var{extent} from its buffer or string.  If
@var{extent} has the @code{duplicable} property, its detachment is
tracked by the undo mechanism.  @xref{Duplicable Extents}.
@end defun

@defun extent-detached-p extent
This function returns @code{nil} if @var{extent} is detached, and
@code{t} otherwise.
@end defun

@defun copy-extent extent &optional object
This function makes a copy of @var{extent}.  It is initially detached.
Optional argument @var{object} defaults to @var{extent}'s object
(normally a buffer or string, but could be @code{nil}).
@end defun

@defun insert-extent extent &optional start end no-hooks object
This function inserts @var{extent} from @var{start} to @var{end} in
@var{object} (a buffer or string).  If @var{extent} is detached from a
different buffer or string, or in most cases when @var{extent} is
already attached, the extent will first be copied as if with
@code{copy-extent}.  This function operates the same as if @code{insert}
were called on a string whose extent data calls for @var{extent} to be
inserted, except that if @var{no-hooks} is non-@code{nil},
@var{extent}'s @code{paste-function} will not be invoked.
@xref{Duplicable Extents}.
@end defun

@node Extent Parents
@section Extent Parents
@cindex extent parent
@cindex extent children
@cindex parent, of extent
@cindex children, of extent

  An extent can have a parent extent set for it.  If this is the case,
the extent derives all its properties from that extent and has no
properties of its own.  The only ``properties'' that the extent keeps
are the buffer or string it refers to and the start and end points.
(More correctly, the extent's own properties are shadowed.  If you
later change the extent to have no parent, its own properties will
become visible again.)

  It is possible for an extent's parent to itself have a parent,
and so on.  Through this, a whole tree of extents can be created,
all deriving their properties from one root extent.  Note, however,
that you cannot create an inheritance loop -- this is explicitly
disallowed.

  Parent extents are used to implement the extents over the modeline.

@defun set-extent-parent extent parent
This function sets the parent of @var{extent} to @var{parent}.
If @var{parent} is @code{nil}, the extent is set to have no parent.
@end defun

@defun extent-parent extent
This function return the parents (if any) of @var{extent}, or
@code{nil}.
@end defun

@defun extent-children extent
This function returns a list of the children (if any) of @var{extent}.
The children of an extent are all those extents whose parent is that
extent.  This function does not recursively trace children of children.
@end defun

@defun extent-descendants extent
This function returns a list of all descendants of @var{extent},
including @var{extent}.  This recursively applies @code{extent-children}
to any children of @var{extent}, until no more children can be found.
@end defun

@node Duplicable Extents
@section Duplicable Extents
@cindex duplicable extent
@cindex unique extents
@cindex extent replica
@cindex extent, duplicable
@cindex extent, unique

  If an extent has the @code{duplicable} property, it will be copied into
strings, so that kill, yank, and undo commands will restore or copy it.

Specifically:

@itemize @bullet
@item
When a string is created using @code{buffer-substring} or
@code{buffer-string}, any duplicable extents in the region corresponding
to the string will be copied into the string (@pxref{Buffer
Contents}).  When the string in inserted into a buffer using
@code{insert}, @code{insert-before-markers}, @code{insert-buffer} or
@code{insert-buffer-substring}, the extents in the string will be copied
back into the buffer (@pxref{Insertion}).  The extents in a string can,
of course, be retrieved explicitly using the standard extent primitives
over the string.

@item
Similarly, when text is copied or cut into the kill ring, any duplicable
extents will be remembered and reinserted later when the text is pasted
back into a buffer.

@item
When @code{concat} is called on strings, the extents in the strings are
copied into the resulting string.

@item
When @code{substring} is called on a string, the relevant extents
are copied into the resulting string.

@item
When a duplicable extent is detached by @code{detach-extent} or string
deletion, or inserted by @code{insert-extent} or string insertion, the
action is recorded by the undo mechanism so that it can be undone later.
Note that if an extent gets detached and then a later undo causes the
extent to get reinserted, the new extent will not be `eq' to the original
extent.

@item
Extent motion, face changes, and attachment via @code{make-extent} are
not recorded by the undo mechanism.  This means that extent changes
which are to be undo-able must be performed by character editing, or by
insertion and detachment of duplicable extents.

@item
A duplicable extent's @code{copy-function} property, if non-@code{nil},
should be a function, and will be run when a duplicable extent is about
to be copied from a buffer to a string (or the kill ring).  It is called
with three arguments: the extent and the buffer positions within it
which are being copied.  If this function returns @code{nil}, then the
extent will not be copied; otherwise it will.

@item
A duplicable extent's @code{paste-function} property, if non-@code{nil},
should be a function, and will be run when a duplicable extent is about
to be copied from a string (or the kill ring) into a buffer.  It is
called with three arguments: the original extent and the buffer
positions which the copied extent will occupy. (This hook is run after
the corresponding text has already been inserted into the buffer.) Note
that the extent argument may be detached when this function is run.  If
this function returns @code{nil}, no extent will be inserted.
Otherwise, there will be an extent covering the range in question.

  Note: if the extent to be copied is already attached to the buffer and
overlaps the new range, the extent will simply be extended and the
@code{paste-function} will not be called.
@end itemize

@node Extents and Events
@section Interaction of Extents with Keyboard and Mouse Events

  If an extent has the @code{mouse-face} property set, it will be
highlighted when the mouse passes over it.  Highlighting is accomplished
by merging the extent's face with the face or faces specified by the
@code{mouse-face} property.  The effect is as if a pseudo-extent with
the @code{mouse-face} face were inserted after the extent in the display
order (@pxref{Extent Endpoints}, display order).

@defvar mouse-highlight-priority
This variable holds the priority to use when merging in the highlighting
pseudo-extent.  The default is 1000.  This is purposely set very high
so that the highlighting pseudo-extent shows up even if there are other
extents with various priorities at the same location.
@end defvar

  You can also explicitly cause an extent to be highlighted.  Only one
extent at a time can be highlighted in this fashion, and any other
highlighted extent will be de-highlighted.

@defun highlight-extent extent &optional highlight-p
This function highlights (if @var{highlight-p} is non-@code{nil}) or
de-highlights (if @var{highlight-p} is @code{nil}) @var{extent}, if
@var{extent} has the @code{mouse-face} property. (Nothing happens if
@var{extent} does not have the @code{mouse-face} property.)
@end defun

@defun force-highlight-extent extent &optional highlight-p
This function is similar to @code{highlight-extent} but highlights
or de-highlights the extent regardless of whether it has the
@code{mouse-face} property.
@end defun

  If an extent has a @code{keymap} property, this keymap will be
consulted for mouse clicks on the extent and keypresses made while
@code{point} is within the extent.  The behavior of mouse clicks and
keystrokes not defined in the keymap is as normal for the buffer.

@node Atomic Extents
@section Atomic Extents
@cindex atomic extent

  If the Lisp file @file{atomic-extents} is loaded, then the atomic
extent facility is available.  An @dfn{atomic extent} is an extent for
which @code{point} cannot be positioned anywhere within it.  This
ensures that when selecting text, either all or none of the extent is
selected.

  To make an extent atomic, set its @code{atomic} property.