Thanks, all, for the responses; it was driving me nuts! I know the game
as "cows and bulls" (or perhaps that's because I'm of British
origin).
I like Dennis' observation: "On the other hand, suid these days seems to
be a fairly blunt and dangerous instrument". I've often believed that if
you thought you needed set-uid (esp. to root!) you can probably get away
with set-gid instead.
-- Dave