intros l1 l2 l3 a H'; elim H'; auto.
intros l4 H'0 H'1 H'2.
cut (canonical A0 eqA ltM l4);
try apply canonical_imp_canonical with (a := a); auto;
intros C0.
apply
canonical_pX_eqT
with
(a := multTerm (A:=A) multA (n:=n) (invTerm (A:=A) invA (n:=n) (T1 A1 n))
a); auto.
change
(canonical A0 eqA ltM
(mults (A:=A) multA (n:=n) (invTerm (A:=A) invA (n:=n) (T1 A1 n))
(pX a l4))) in |- *; auto.
apply (eqT_sym A n);
apply (eqT_trans A n) with (y := multTerm (A:=A) multA (n:=n) (T1 A1 n) a);
auto.
apply (T1_eqT A A1 eqA); auto.
apply canonical_nzeroP with (ltM := ltM) (p := l4); auto.
intros a1 a2 l4 l5 l6 H'0 H'1 H'2 H'3 H'4 H'5.
apply canonical_cons; auto.
apply (canonical_pX_order A A0 eqA) with (ltM := ltM) (l := l4); auto.
apply canonical_nzeroP with (ltM := ltM) (p := pX a1 l4); auto.
intros a1 a2 l4 l5 l6 H'0 H'1 H'2 H'3 H'4 H'5 H'6.
apply H'1; auto.
apply canonical_skip_fst with (b := a1); auto.
apply canonical_skip_fst with (b := a2); auto.
intros a1 a2 l4 l5 l6 H'0 H'1 H'2 H'3 H'4 H'5 H'6.
apply canonical_cons; auto.
apply minusTerm_ltT_l; auto.
apply (canonical_pX_order A A0 eqA) with (l := l4); auto.
apply canonical_nzeroP with (ltM := ltM) (p := pX a1 l4); auto.
intros a1 a2 l4 l5 l6 H'0 H'1 H'2 H'3 H'4 H'5.
apply canonical_cons; auto.
apply invTerm_ltT_l; auto.
apply (canonical_pX_order A A0 eqA) with (l := l5); auto.
apply canonical_nzeroP with (ltM := ltM) (p := pX a1 l4); auto.
Qed.

Theorem canonical_minusP :
forall l1 l2 l3 : list (Term A n),
minusP l1 l2 l3 ->
canonical A0 eqA ltM l1 ->
canonical A0 eqA ltM l2 -> canonical A0 eqA ltM l3.