Fixpoint exprlist_app (rl1 rl2: exprlist) : exprlist :=
  match rl1 with
  | Enil => rl2
  | Econs r1 rl1' => Econs r1 (exprlist_app rl1' rl2)
  end.

(* Specifications: *)
Lemma exprlist_app_assoc: forall rl2 rl3 rl1, exprlist_app (exprlist_app rl1 rl2) rl3 = exprlist_app rl1 (exprlist_app rl2 rl3).