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).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).

Lemma commut_subterm_R_exp : commut value subterm R_exp.