Definition subst := A -> (A -> val) -> val.Definition stack_subst (s: stack A val) (sub: subst) : stack A val :=
    fun x => sub x s.Definition apply_subst {B}
          (e : (A -> val) -> B) (sub : subst) : (A -> val) -> B :=
    fun s => e (stack_subst s sub).Definition substlist := list (A * @expr A val).(* The identity substitution *)
  Definition subst0 : subst := fun x => stack_get x.(* Substitution of one variable *)
  Definition subst1 e x : subst :=
    fun x' => if x' ?[ eq ] x then e else stack_get x'.(* Truncating substitution of one variable *)
  Definition subst1_trunc e x : subst :=
    fun x' => if x' ?[ eq ] x then e else empty_open.(* Simultaneous substitution of a finite list of variables *)
  Fixpoint substl_aux (subs: substlist) : subst :=
    match subs with
      | nil => subst0
      | cons (x,e) subs' => fun x' => 
        if x' ?[ eq ] x then e else substl_aux subs' x'
    end.(* Truncating simultaneous substitution of a finite list of variables *)
  Fixpoint substl_trunc_aux (subs: substlist) : subst :=
    match subs with
      | nil => fun _ => empty_open
      | cons (x,e) subs' => fun x' => 
        if x' ?[ eq ] x then e else substl_trunc_aux subs' x'
    end.Definition substl es v := substl_aux es v.Definition substl_trunc es v := substl_trunc_aux es v.(* Binds tighter than function application *)
(* We have the e at precedence level one less than the precedence of the infix
   // operator to make parsing work. *)

Notation "g [{ e // x }]" := (apply_subst g (subst1 e x)) (at level 9, e at level 39,
  format "g [{ e // x }]") : open_scope.Notation "g [{ e //! x }]" := (apply_subst g (subst1_trunc e x)) (at level 9, e at level 39,
  format "g [{ e //! x }]") : open_scope.Notation " g '//' es " := (apply_subst g (substl es)) (at level 40) : open_scope.Notation " g '//!' es " := (apply_subst g (substl_trunc es)) (at level 40) : open_scope.

Lemma subst0_id B (g: @open A val B) : (apply_subst g (@subst0 A val)) = g.