intros l1 l2 a H' H'0.
apply order_plusP with (1 := os) (plusA := plusA) (l1 := l1) (l2 := l2); auto.
apply pluspf_is_plusP; auto.
apply canonical_pluspf; auto.
apply canonical_imp_canonical with (a := a); auto.
apply canonical_imp_canonical with (a := a); auto.
Qed.
Hint Resolve order_pluspf.

Theorem order_minuspf :
forall (l1 l2 : list (Term A n)) (a : Term A n),
canonical A0 eqA ltM (pX a l1) ->
canonical A0 eqA ltM (pX a l2) ->
canonical A0 eqA ltM (pX a (minuspf l1 l2)).