intros l1 l2 l3 a H'; generalize a; elim H'; auto.
intros a1 a2 l4 l5 l6 H'0 H'1 H'2 a0 H'3 H'4 H'5.
apply canonical_cons; auto.
apply (canonical_pX_order _ A0 eqA) with (l := l4); auto.
apply (canonical_nzeroP _ A0 eqA _ ltM) with (p := pX a1 l4); auto.
intros a1 a2 l4 l5 l6 H'0 H'1 H'2 H'3 a0 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 a0 H'4 H'5 H'6.
apply canonical_cons; auto.
apply ltT_eqTr with (a := a1); auto.
apply (eqT_sym A n (plusTerm (A:=A) plusA (n:=n) a1 a2)).
apply plusTerm_eqT1; auto.
apply (canonical_pX_order _ A0 eqA) with (l := l4); auto.
apply (canonical_nzeroP _ A0 eqA _ ltM) with (p := pX a1 l4); auto.
intros a1 a2 l4 l5 l6 H'0 H'1 H'2 a0 H'3 H'4 H'5.
apply canonical_cons; auto.
apply (canonical_pX_order _ A0 eqA) with (l := l5); auto.
apply (canonical_nzeroP _ A0 eqA _ ltM) with (p := pX a1 l4); auto.
Qed.

Theorem canonical_plusP :
forall l1 l2 l3,
plusP l1 l2 l3 ->
canonical A0 eqA ltM l1 ->
canonical A0 eqA ltM l2 -> canonical A0 eqA ltM l3.