I believe you need:

a b c d
(a b c) d
a (b c d)
a b (c d)
(a b) (c d)
(a b) c d

to find all solutions. (Sorry, I haven't verified that you absolutely need a b c d. Never thought of that. Howard what made you think of that combo.)

[ 25. September 2002, 02:08: Message edited by: Jack Lothian ]
_________________________
Jack