auto.
Qed.

Lemma al2_t :
forall (t : nat) (di0 cx0 al0 : BV) (mem0 : Memo) (ad0 da0 : BV),
al2 (S t) di0 cx0 al0 mem0 ad0 da0 =
match IsNull (cx2 t di0 cx0 al0 mem0 ad0 da0) return BV with
| true => al2 t di0 cx0 al0 mem0 ad0 da0
| false => al2 t di0 cx0 al0 mem0 ad0 da0
end.