auto.
Qed.

Lemma cx_t :
forall (t : nat) (di0 cx0 al0 : BV) (mem0 : Memo),
cx (S t) di0 cx0 al0 mem0 =
match IsNull (cx t di0 cx0 al0 mem0) return BV with
| true => cx t di0 cx0 al0 mem0
| false => BV_decrement (cx t di0 cx0 al0 mem0)
end.