The "umount" command has a "-f" option that's not
documented in the manpage.
% umount --help
Usage: umount [-hV]
umount -a [-f] [-r] [-n] [-v] [-t vfstypes]
umount [-f] [-r] [-n] [-v] special | node...
I have verified this to be true.
The -f is not documented because it's not available on all platforms
since the functionality of the flag depends on the existence of a