About setf extensions

[Xue Fuqiao]

In (info "(cl) Setf Extensions"):

     Most of these have directly corresponding "set" functions, like
     `use-local-map' for `current-local-map', or `goto-char' for
     `point'.  A few, like `point-min', expand to longer sequences of
     code when they are used with `setf' (`(narrow-to-region x
     (point-max))' in this case).

I don't understand the `point-min' example.  What does the "x" mean
here?  I know it is the START argument of `narrow-to-region', but where
is it from?  And where will it be used?  Thanks.

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao