Document 'split-frame' and 'merge-frames' in Elisp manual

* doc/lispref/frames.texi (Splitting and Merging Frames): New
section.
* doc/lispref/elisp.texi (Top): Add new section.
* doc/lispref/windows.texi (Basic Windows): Explain terms
'layout' and 'identity' of windows.
(Window Configurations): Explain that window identities are
preserved by window configurations and get lost with window
states.

doc/lispref/elisp.texi

@@ -1137,6 +1137,7 @@ Frames
 * Frame Titles::            Automatic updating of frame titles.
 * Deleting Frames::         Frames last until explicitly deleted.
 * Finding All Frames::      How to examine all existing frames.
+* Splitting and Merging Frames:: Transferring window layouts between frames.
 * Minibuffers and Frames::  How a frame finds the minibuffer to use.
 * Input Focus::             Specifying the selected frame.
 * Visibility of Frames::    Frames may be visible or invisible, or icons.

doc/lispref/frames.texi

@@ -151,6 +151,7 @@ unique id.
 * Frame Titles::                Automatic updating of frame titles.
 * Deleting Frames::             Frames last until explicitly deleted.
 * Finding All Frames::          How to examine all existing frames.
+* Splitting and Merging Frames:: Transferring window layouts between frames.
 * Minibuffers and Frames::      How a frame finds the minibuffer to use.
 * Input Focus::                 Specifying the selected frame.
 * Visibility of Frames::        Frames may be visible or invisible, or icons.
@@ -3076,6 +3077,85 @@ criteria, and should return non-@code{nil} if the frame satisfies the
 criteria.
 @end defun
 
+
+@node Splitting and Merging Frames
+@section Splitting and Merging Frames
+@cindex splitting a frame
+@cindex merging two frames
+@cindex Transferring window layouts
+
+The following two functions are useful to transfer parts of the window
+layout (@pxref{Basic Windows}) of one frame to two separate frames and
+to merge the window layouts of two separate frames into one frame.
+
+@deffn Command split-frame &optional frame arg
+This command transfers some windows of the specified @var{frame} to a
+new frame and subsequently deletes them on @var{frame}.  @var{frame}
+must be a live frame and defaults to the selected frame.  @var{arg}
+specifies the number of windows to transfer and defaults to 1.
+Interactively, @var{arg} is the prefix argument.
+
+In a first step, it divides the child windows (@pxref{Windows and
+Frames}) of @var{frame}'s main window (@pxref{Side Windows}) into two
+parts.  The first part includes the first @var{arg} child windows if
+@var{arg} is positive, and the -@var{arg} last child windows if
+@var{arg} is negative.  The second part includes the remaining child
+windows of @var{frame}'s main window.
+
+In a second step, it clones (@pxref{Window Configurations}) into a newly
+created frame each of the windows of the part which does not include
+@var{frame}'s selected window (@pxref{Selecting Windows}) and removes
+those windows from @var{frame}.  Note that the original identity
+(@pxref{Basic Windows}) of the cloned windows is lost in this step.  The
+identity of the windows retained on @var{frame} is preserved, however.
+In a final step, it deletes the windows that have been cloned from
+@var{frame} and returns the newly created frame.
+
+This function signals an error if @var{arg} is either zero or not a
+number, or if @var{frame}'s main window is either live or does not have
+more child windows than specified by the absolute value of @var{arg}.
+@end deffn
+
+@deffn Command merge-frames &optional frame1 frame2 vertical
+This command transfers the main window (@pxref{Side Windows}) of
+@var{frame2} to @var{frame1}.  Both @var{frame1} and @var{frame2} must
+specify live frames.
+
+In a first step it splits the main window of @var{frame1}.  If
+@var{vertical} is non-@code{nil}, it makes the new window below the old
+main window of @var{frame1}.  Otherwise, it makes the new window on the
+right of @var{frame1}'s main window.
+
+In a second step it makes the new window a clone (@pxref{Window
+Configurations}) of the main window of @var{frame2}.  The original
+identity (@pxref{Basic Windows}) of the cloned windows is lost in that
+step.  In a final step, it deletes @var{frame2} if the merge completed
+successfully and returns @var{frame1}.
+
+Interactively, @var{vertical} is the prefix argument, @var{frame1} is
+the selected frame and @var{frame2} is the frame following @var{frame1}
+in the list of all frames (@pxref{Finding All Frames}).
+@end deffn
+
+In general you cannot ``undo'' a @code{split-frame} call with a
+subsequent @code{merge-frames} call for the following reasons: In its
+second step @code{merge-frame} puts the main window of @var{frame2} into
+an internal window split off in the first step.  No such window was
+present in the configuration of @var{frame1} before calling
+@code{split-frame}.  Also, after merging is complete, the window cloned
+from the main window of @var{frame2} will occupy as much space as the
+main window of @var{frame1} before merging.  This might not match the
+space distribution of the original layout.
+
+Hence, if your preferred workflow is to temporarily split off windows
+for working on them in a separate frame and later continue with the
+original layout of the original frame, you should proceed as follows:
+Save the configuration (@pxref{Window Configurations}) of the original
+frame, call @code{split-frame} to create the separate frame and finally
+restore the saved configuration of the original frame and delete the
+separate frame.
+
+
 @node Minibuffers and Frames
 @section Minibuffers and Frames
 

doc/lispref/windows.texi

@@ -78,14 +78,19 @@ disambiguation, we use the term @dfn{window-system window} when we mean
 the window-system window corresponding to an Emacs frame.
 
 @cindex tiled windows
+@cindex window layout
   Unlike X windows, Emacs windows are @dfn{tiled}; they never overlap
 within the area of their frame.  When a window is created, resized, or
 deleted, the change in window space is taken from or given to other
 windows on the same frame, so that the total area of the frame is
-unchanged.
+unchanged.  The @dfn{window layout} of a frame is the result of all
+tiling operations for that frame.
 
+@cindex window object
+@cindex window identity
 In Emacs Lisp, windows are represented by a special Lisp object type
-(@pxref{Window Type}).
+(@pxref{Window Type}).  Two windows are identic if and only if they are
+represented by the same Lisp object.
 
 @defun windowp object
 This function returns @code{t} if @var{object} is a window (whether or
@@ -7263,11 +7268,16 @@ and/or two columns.
 In the context of window states, the @dfn{clone of a window} is a window
 that has the same decorations and contents as the window whose state was
 used to produce it, but is actually represented by another window
-object.  Operating on the original or the clone of a window does not
-affect the other in any way.  Note that while @code{window-state-get}
-clones existing windows, these clones are not yet valid windows.  They
-become valid only after @code{window-state-put} has put them into a live
-frame.
+object.  Hence it does not have the same identity as the original
+window.  Operating on the original or the clone of a window does not
+affect the other in any way.
+
+Note that while @code{set-window-configuration} always restores the
+identity of the windows it puts back into their frame,
+@code{window-state-put} never does that.  Note also that while
+@code{window-state-get} clones existing windows, these clones are not
+yet valid windows.  They become valid only after @code{window-state-put}
+has put them into a live frame.
 
 By default, @code{set-window-configuration} and @code{window-state-put}
 may delete a window from the restored configuration or state when they