intros p; elim p; clear p; auto.
simpl in |- *; intros q a b H' H'0.
cut (canonical A0 eqA ltM q);
[ intros Op0 | apply canonical_imp_canonical with (a := b) ];
auto.
exists (mults (A:=A) multA (n:=n) a q); simpl in |- *; auto.
rewrite <- pO_pluspf_inv2; auto.
intros a1 p H' q a0 b H'0 H'1; auto.
elim (H' q a1 b); simpl in |- *; [ intros c E | idtac | idtac ]; auto.
2: apply canonical_imp_canonical with (a := a0); auto.
rewrite E.
exists
(pluspf (A:=A) A0 (eqA:=eqA) plusA eqA_dec (n:=n) (ltM:=ltM) ltM_dec
(mults (A:=A) multA (n:=n) a0 q)
(pX (multTerm (A:=A) multA (n:=n) a1 b) c)); auto.
rewrite <- pluspf_inv1_eq; auto.
apply multTerm_ltT_r; auto.
apply (canonical_pX_order _ A0 eqA) with (l := p); auto.
Qed.

Theorem in_multpf_head :
forall (p q : list (Term A n)) (a b : Term A n),
canonical A0 eqA ltM (pX a p) ->
canonical A0 eqA ltM (pX b q) ->
In (multTerm (A:=A) multA (n:=n) a b) (multpf (pX a p) (pX b q)).