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).