proposition: Lemma pair_card x y : card {{x, y}} = 2.
proof: Proof. unfold card, pair, insert. destruct (card_def ({{x}} + {{y}})); simpl. rewrite <- e. rewrite multiset2list_cons_length. destruct (multiset2list_singleton x) as [x' [_ xx']]. rewrite xx'; trivial. Qed.
imports: Coq(List Setoid Morphisms) CoLoR(RelExtras MultisetCore ListPermutation NatUtil ListExtras LogicUtil)