Definition whnf_bd_ord (x y : approx) :=
    lexprod value (fun _ : value => value) (R_exp beta_delta)
      (fun _ : value => subterm) (f1 x) (f1 y).

Theorem beta_delta_whnf : forall (e : env) (t : term), sn (ctxt beta_delta) e t -> {u : term | red beta_delta e t u & head_normal beta_delta e u}.