proposition: Lemma equal_or_subset_and_not_equal_subset_b {x y} : (equal x y || (subset x y && negb (equal x y))) = subset x y.
proof: Proof. to_caps; bool_congr_setoid; to_caps; intuition; try fsetdec. Qed.
imports: Coq.MSets.MSetInterface Coq.MSets.MSetProperties Coq.MSets.MSetFacts Coq.MSets.MSetDecide Fiat.Common.Instances Fiat.Common.BoolFacts Fiat.Common