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Around-advice lets you “wrap” a Lisp expression “around” the
original function definition. You specify where the original function
definition should go by means of the special symbol ad-do-it.
Where this symbol occurs inside the around-advice body, it is replaced
with a progn containing the forms of the surrounded code. Here
is an example:
(defadvice foo (around foo-around)
"Ignore case in `foo'."
(let ((case-fold-search t))
ad-do-it))
Its effect is to make sure that case is ignored in
searches when the original definition of foo is run.
This is not really a variable, but it is somewhat used like one in around-advice. It specifies the place to run the function's original definition and other “earlier” around-advice.
If the around-advice does not use ad-do-it, then it does not run
the original function definition. This provides a way to override the
original definition completely. (It also overrides lower-positioned
pieces of around-advice).
If the around-advice uses ad-do-it more than once, the original
definition is run at each place. In this way, around-advice can execute
the original definition (and lower-positioned pieces of around-advice)
several times. Another way to do that is by using ad-do-it
inside of a loop.