check-duality (a b)
Returns (values dual? non-duality-info contradictory?)
dual? is non-NIL if a and b form dual sets of clauses.
contradictory? is non-NIL if a and b are contradictory and hence not dual.
non-duality-info: a missing clause if they are not dual and not contraditory, a pair of clauses that contradict, or NIL otherwise.
Check duality of two monotone boolean functions.