proposition: Theorem rm_incl: forall l1 l2, incl (rm l1 l2) l2.
proof: Proof. intros l1; elim l1; simpl; auto with datatypes; clear l1. intros a l1 Rec l2; elim l2; simpl; auto with datatypes; clear l2. intros b l2 H; case_eq (weight a b); auto with datatypes. Qed.
imports: List Permutation UList