Definition subst_substlist (es fs : @substlist A val) : substlist :=
    map (fun x => (fst x, (snd x) // fs)) es.

(* Specifications: *)
Lemma subst_combine {B} (e : @open A val B) (es fs : substlist) : (e // es // fs) = (e // (app (subst_substlist es fs) fs)).