intros a p q; try assumption.
apply (eqp_trans _ _ _ _ _ _ _ _ _ cs n) with (y := minuspf p q); auto.
apply (eqp_sym _ _ _ _ _ _ _ _ _ cs n).
apply minuspf_inv3a; auto.
Qed.
Hint Resolve canonical_minuspf.

Theorem pluspf_minuspf_id :
forall p q : list (Term A n),
canonical A0 eqA ltM p ->
canonical A0 eqA ltM q ->
eqP A eqA n
(pluspf (A:=A) A0 (eqA:=eqA) plusA eqA_dec (n:=n) (ltM:=ltM) ltM_dec
(minuspf p q) q) p.