About the overlay property `modification-hooks'

[Xue Fuqiao]

In (info "(elisp) Overlay Properties"):

     When called after a change, each function receives five arguments:
     the overlay, `t', the beginning and end of the text range just
     modified, and the length of the pre-change text replaced by that
     range.  (For an insertion, the pre-change length is zero; for a
     deletion, that length is the number of characters deleted, and the
     post-change beginning and end are equal.)

I'm perplexed about the sentence in the parentheses.  What does the
"post-change beginning and end" mean here?  Can anybody help?  Thanks.

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