Force file completion when hitting TAB twice

[Yuri D'Elia <wavexx@xxxxxxxxxxxx>]

Hi everyone. I'm trying to customize the completion of zsh so that:

* when I hit TAB the default _complete completer is used.
* if no completions are available, I get a beep.
* if I hit TAB again, or hit TAB twice, I get a _files completer
  (offering me to complete filenames regardless of the context).

I would *not* like to change the default completer to complete files if
no other one is available.

How can I achieve that?

The nearest thing I was able to find is this:

  http://www.zsh.org/mla/users/1999/msg00051.html

Thanks for any help.