Hint Resolve agree_concat : agree.
(* Concatenation and insertion commute. *)

(* Specifications: *)
Lemma insert_concat:
forall (A : Type) n x nx (o : option A) e1 e2,
length e2 = n ->
n + x = nx ->
raw_insert nx o (concat e1 e2) = concat (raw_insert x o e1) e2.