what are qmail-clean commands?

[Wayne Chu]

I am unable to clear the queue.
The MAN page of qmail-clean simply says:

     NAME
          qmail-clean - clean up the queue directory

     SYNOPSIS
          qmail-clean

     DESCRIPTION
          qmail-clean reads a cleanup command from descriptor 0,
          performs the cleanup, prints the results to descriptor 1,
          and repeats.

     SEE ALSO
          qmail-send(8)

but what are these cleanup commands? where can I find them?
thanks