proposition: Lemma test_not_In_disj : forall x y s1 s2 s3 s4, ~ In x (union s1 (union s2 (union s3 (add y s4)))) -> ~ (In x s1 \/ In x s4 \/ E.eq y x).
proof: Proof. fsetdec. Qed.
imports: Decidable Setoid DecidableTypeEx MSetFacts