(* Applying a transformation to an environment does not affect its length. *)

(* Specifications: *)
Lemma length_map_general:
forall A B (f : A -> B) (e : env A),
length (map f e) = length e.
Lemma length_map:
forall A B (f : A -> B) (e : env A) k,
length e <= k ->
length (map f e) <= k.