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(emacs-lisp-intro.info)append interactive

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The `append-to-buffer' Interactive Expression
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   Since the `append-to-buffer' function will be used interactively,
the function must have an `interactive' expression.  (For a review of
`interactive', see Note: Making a Function Interactive.)
The expression reads as follows:

     (interactive "BAppend to buffer: \nr")

This expression has an argument inside of quotation marks and that
argument has two parts, separated by `\n'.

   The first part is `BAppend to buffer: '.  Here, the `B' tells Emacs
to ask for the name of the buffer that will be passed to the function.
Emacs will ask for the name by prompting the user in the minibuffer,
using the string following the `B', which is the string `Append to
buffer: '.  Emacs then binds the variable `buffer' in the function's
argument list to the specified buffer.

   The newline, `\n', separates the first part of the argument from the
second part.  It is followed by an `r' that tells Emacs to bind the two
arguments that follow the symbol `buffer' in the function's argument
list (that is, `start' and `end') to the values of point and mark.