What do undocumented -fstype and -prune options to find do?

[Dominic Dunlop]

Running strings across the A/UX 2.0 find command (the way one does), I
spot two undocumented options:

-fstype	Appears only to traverse mount points if mounted filesystem is
	of the named type.  What names are acceptable?  I don't know.

-prune	Apparently taken from  a fairly recent draft of the 1003.2 shell
	and tools standard: ``Causes find to stop its descent from that
	point in the file hierarchy.''  Huh?  What use is this, given
	that -prune does NOT take an argument naming the directory at
	which to stop, or a number giving the number of levels to
	descend.

Would anybody care to elucidate?

(I was looking for BSD's -xdev or System V.3's -mount and/or -local.  No
luck.)
-- 
Dominic Dunlop