proposition: Lemma prefix_length: forall ys xs, prefix ys xs -> length ys <= length xs.
proof: Proof. Proof using. intros ys xs [ zs ? ]. subst xs. rew_list. math. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation LibList)