What does a minus sign mean in /etc/pam.d/passwd?

[Jonathan Ryshpan]

The standard passwd file for Fedora-11 has a line starting with a minus
sign:
        $ cat /etc/pam.d/passwd
        #%PAM-1.0
        auth       include	system-auth
        account    include	system-auth
        password   substack	system-auth
        -password   optional	pam_gnome_keyring.so
What does this mean?

This has come up with starting gnome-keyring at login time.  
The Gnome Project advice page:
        http://live.gnome.org/GnomeKeyring/Pam
recommends that to have the gnome-keyring authorized at login time, the
passwd file should end with a line like the one at the end of this file
(among other things), but without the minus sign.  I can't find
documentation on this use of a minus sign.

Thanks - jon