intros a; case a; simpl in |- *; unfold LetP in |- *; auto.
intros; apply minuspf_refl with (1 := cs); auto.
Qed.

Theorem spolyf_com :
forall (a b : list (Term A n)) (Ca : canonical A0 eqA ltM a)
(Cb : canonical A0 eqA ltM b),
eqP A eqA n (spolyf a b Ca Cb)
(mults (A:=A) multA (n:=n) (invTerm (A:=A) invA (n:=n) (T1 A1 n))
(spolyf b a Cb Ca)).