Proof with norm.
intros.
apply pred_ext.
apply andp_right. apply andp_left2... apply andp_left1...
apply andp_right. apply andp_left2... apply andp_left1...
Qed.

Lemma andp_assoc {A} {NA: NatDed A} : forall P Q R : A,
(P && Q) && R = P && (Q && R).