zipsort.icn: Program to sort mailing labels by ZIP code

   This program sorts labels produced by labels in ascending
order of their postal zip codes.

Option:

   The option -d n sets the number of lines per label to n.
The default is 9. This value must agree with the value used to
format the labels.

Zip Codes:

   The zip code must be the last nonblank string at the
end of the label.  It must consist of digits but may have an
embedded dash for extended zip codes.  If a label does not end
with a legal zip code, it is placed after all labels with legal
zip codes.  In such a case, an error messages also is written to
standard error output.