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(emacs-lisp-intro.info)append-to-buffer overview

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An Overview of `append-to-buffer'
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   The `append-to-buffer' command uses the `insert-buffer-substring'
function to copy the region.  `insert-buffer-substring' is described by
its name: it takes a string of characters from part of a buffer, a
"substring", and inserts them into another buffer.  Most of
`append-to-buffer' is concerned with setting up the conditions for
`insert-buffer-substring' to work: the code must specify both the
buffer to which the text will go and the region that will be copied.
Here is the complete text of the function:

     (defun append-to-buffer (buffer start end)
       "Append to specified buffer the text of the region.
     It is inserted into that buffer before its point.
     
     When calling from a program, give three arguments:
     a buffer or the name of one, and two character numbers
     specifying the portion of the current buffer to be copied."
       (interactive "BAppend to buffer: \nr")
       (let ((oldbuf (current-buffer)))
         (save-excursion
           (set-buffer (get-buffer-create buffer))
           (insert-buffer-substring oldbuf start end))))

   The function can be understood by looking at it as a series of
filled-in templates.

   The outermost template is for the function definition.  In this
function, it looks like this (with several slots filled in):

     (defun append-to-buffer (buffer start end)
       "DOCUMENTATION..."
       (interactive "BAppend to buffer: \nr")
       BODY...)

   The first line of the function includes its name and three arguments.
The arguments are the `buffer' to which the text will be copied, and
the `start' and `end' of the region in the current buffer that will be
copied.

   The next part of the function is the documentation, which is clear
and complete.