proposition: Lemma drop_drop {A} x y (ls : list A) : drop x (drop y ls) = drop (y + x) ls.
proof: Proof. revert x y. induction ls as [|l ls IHls]. { intros [|x] [|y]; reflexivity. } { intros x [|y]; simpl. { reflexivity. } { apply IHls. } } Defined.
imports: Fiat.Common.Coq__8_4__8_5__Compat Coq.ZArith.ZArith Coq.Lists.List Coq.Lists.SetoidList Coq.Bool.Bool Fiat.Common Fiat.Common.List.Operations Fiat.Common.Equality Fiat.Common.List.FlattenList Fiat.Common.LogicFacts