Hint Resolve insert_concat length_replicate insert_concat_replicate
concat_replicate_is_iterated_insert : insert_concat.
(* A special case of [length_concat]. *)

Lemma length_concat_replicate:
forall A (a : A) (e1 : env A) n1 n2 n,
length e1 <= n1 ->
n1 + n2 = n ->
length (concat e1 (replicate n2 a)) <= n.