About `constrain-to-field'

[Xue Fuqiao]

I have read (info "(elisp) Fields") and doc string of the function
`constrain-to-field', but I cannot understand what this function does,
especially for its optional arguments.

I've searched the archive of this mailing list, but the only two
results are about whitespace characters removing.  Maybe I need
example(s) and some explanation.

-- 
Xue Fuqiao
http://www.gnu.org/software/emacs/