Example
=======
compctl -u -x 's[+] c[-1,-f],s[-f+]' \
-g '~/Mail/*(:t)' - 's[-f],c[-1,-f]' -f -- mail
This is to be interpreted as follows:
If the current command is mail, then
if ((the current word begins with + and the previous word is -f)
or (the current word begins with -f+)), then complete the
non-directory part (the `:t' glob modifier) of files in the
directory ~/Mail; else
if the current word begins with -f or the previous word was -f,
then complete any file; else
complete user names.