proposition: Lemma cardinal_one_In_same v (H : cardinal v = 1) : forall x y, In x v -> In y v -> E.eq x y.
proof: Proof. cardinal_to_list. intros; in_to_elements; InA_concretitze. fsetdec. Qed.
imports: Coq.MSets.MSetInterface Coq.MSets.MSetProperties Coq.MSets.MSetFacts Coq.MSets.MSetDecide Fiat.Common.Instances Fiat.Common.BoolFacts Fiat.Common