simple destruct a.
intros.
generalize e.
elim t; intros; apply Acc_intro; intros.
inversion_clear H.
inversion_clear H.
inversion_clear H1; auto with pts.
inversion_clear H1; auto with pts.
inversion_clear H1; auto with pts.
Qed.
Inductive R_exp : value -> value -> Prop :=
Rex_intro :
forall (e : env) (x y : term), ctxt R e x y -> R_exp (e, y) (e, x).
Hint Resolve Rex_intro: pts.

Lemma commut_subterm_R_exp : commut value subterm R_exp.
red in |- *.