intros a b p q H' Z; try assumption.
rewrite (minusP_inv3b a b p q (minuspf (pX a p) (pX b q))); auto.
Qed.
Hint Resolve pluspf_inv1 pluspf_inv2 pluspf_inv3a pluspf_inv3b.
Hint Resolve minuspf_inv1 minuspf_inv2 minuspf_inv3a minuspf_inv3b.

Theorem minuspf_comp :
forall p q r s : list (Term A n),
canonical A0 eqA ltM p ->
canonical A0 eqA ltM q ->
canonical A0 eqA ltM r ->
canonical A0 eqA ltM s ->
eqP A eqA n p r ->
eqP A eqA n q s -> eqP A eqA n (minuspf p q) (minuspf r s).