unfold pluspf in |- *; case (plusp (l1, l2)); simpl in |- *; auto.
Qed.
Hint Resolve pluspf_is_plusP.

Theorem order_plusP :
forall l1 l2 l3 a,
plusP 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).