intros p q H'.
apply minusP_pO_is_eqP with (r := minuspf p q); auto.
Qed.

Theorem order_minusP :
forall (l1 l2 l3 : list (Term A n)) (a : Term A n),
minusP l1 l2 l3 ->
canonical A0 eqA ltM (pX a l1) ->
canonical A0 eqA ltM (pX a l2) ->
canonical A0 eqA ltM l3 -> canonical A0 eqA ltM (pX a l3).