proposition: Lemma step_monotonic lss : sizeof_pair (step lss) <= sizeof_pair lss.
proof: Proof. unfold step, sizeof_pair; simpl; apply Nat.add_le_mono; apply length_filter. Qed.
imports: Fiat.Common.Coq__8_4__8_5__Compat Coq.Arith.EqNat Coq.Arith.Compare_dec Coq.ZArith.ZArith Coq.Lists.List Fiat.Common.List.ListFacts Fiat.Common