intros a b p q H' H1';
rewrite (plusP_inv3b a b p q (pluspf (pX a p) (pX b q)));
auto.
Qed.

Theorem order_pluspf :
forall l1 l2 a,
canonical A0 eqA ltM (pX a l1) ->
canonical A0 eqA ltM (pX a l2) -> canonical A0 eqA ltM (pX a (pluspf l1 l2)).