intros a p q; case q; auto.
rewrite <- pO_pluspf_inv2; auto.
rewrite <- pO_pluspf_inv2; auto.
intros a0 l H'0 H'1.
change (pX a (pluspf p (pX a0 l)) = pluspf (pX a p) (pX a0 l)) in |- *.
apply pluspf_inv1_eq; auto.
apply (canonical_pX_order _ A0 eqA) with (l := l); auto.
Qed.

Theorem pluspf_inv2_eqa :
forall a p q,
canonical A0 eqA ltM (pX a p) ->
canonical A0 eqA ltM (pX a q) -> pX a (pluspf p q) = pluspf p (pX a q).