sets implements basic set operators, so that you can easily
do things like
	% grep somestring `sets *.c -not xxx.c`
which greps for somestring in all C files except xxx.c.

The general format is

	% sets e1... op1 e2... op2 e3...

where op's are operators and e's are any elements not in the operator list.
The output is the elements formed by evaluating the operators left-to-right.

The valid operators are
	-i (intersection)
	-u (union)
	-not (remove words from left wordgroup if they are in right wordgroup)