proposition: Lemma union_diff P Q (Q_dec : forall x, {Q x}+{~Q x}) : union (diff P Q) Q [=] union P Q.
proof: Proof. split. fo. unfold union, diff, mem. destruct (Q_dec a); tauto. Qed.
imports: Coq(Basics Morphisms Setoid) CoLoR(LogicUtil RelUtil FunUtil ListUtil NatUtil ClassicUtil BoundNat)