intros p r H'; elim H'; simpl in |- *; auto.
intros ma mb p0 q H'0 H'1 H'2 q0 s H'3 H'4 H'5 H'6 H'7.
cut (canonical A0 eqA ltM p0);
[ intros Op0 | apply canonical_imp_canonical with (a := ma); auto ].
cut (canonical A0 eqA ltM q);
[ intros Op1 | apply canonical_imp_canonical with (a := mb); auto ].
cut (~ zeroP (A:=A) A0 eqA (n:=n) ma);
[ intros Z0 | apply canonical_nzeroP with (ltM := ltM) (p := p0); auto ].
cut (~ zeroP (A:=A) A0 eqA (n:=n) mb);
[ intros Z1 | apply canonical_nzeroP with (ltM := ltM) (p := q) ];
auto.
Qed.

Theorem multpf_com :
forall p q : list (Term A n),
canonical A0 eqA ltM p ->
canonical A0 eqA ltM q -> eqP A eqA n (multpf p q) (multpf q p).