(* Attempting to rewrite in both directions may seem redundant, because of the
   symmetry of the law [insert_insert]. It is not: because [omega] fails in
   the presence of meta-variables, rewriting in one direction may be possible
   while the other direction fails. *)

Ltac insert_insert :=
  first [
    rewrite    insert_insert; [ reflexivity | omega ]
  | rewrite <- insert_insert; [ reflexivity | omega ]
  ].

Lemma insert_eq_insert_2: forall A x a1 a2 (e1 e2 : env A), insert x a1 e1 = insert x a2 e2 -> forall b, insert x b e1 = insert x b e2.