Hint Rewrite lift_lift_retval: norm2.

(* Specifications: *)
Lemma lift_lift_x:  (* generalizes lift_lift_val *)
forall t t' P (v: t),
(@liftx (Tarrow t (LiftEnviron t')) P (@liftx (LiftEnviron t) v)) =
(@liftx (LiftEnviron t') (P v)).
Lemma lift0C_exp {A}{NA: NatDed A}:
forall (B: Type) (f: B -> A), `(exp f) = EX x:B, `(f x).