proposition: Lemma union_subset_2b : forall s s', subset s' (union s s').
proof: Proof. to_caps; auto with sets. Qed.
imports: Coq.MSets.MSetInterface Coq.MSets.MSetProperties Coq.MSets.MSetFacts Coq.MSets.MSetDecide Fiat.Common.Instances Fiat.Common.BoolFacts Fiat.Common