intros a b H' H0;
apply
(eqTerm_trans _ _ _ _ _ _ _ _ _ cs n)
with (y := plusTerm (A:=A) plusA (n:=n) a (invTerm (A:=A) invA (n:=n) b));
auto.
apply (eqTerm_sym _ _ _ _ _ _ _ _ _ cs n);
apply zeroP_plusTerml with (1 := cs); auto.
apply (eqT_trans A n) with (y := b); auto.
Qed.

Theorem minusTerm_zeroP :
forall a b : Term A n,
eqT a b ->
zeroP (A:=A) A0 eqA (n:=n) b ->
eqTerm (A:=A) eqA (n:=n) (minusTerm (A:=A) minusA (n:=n) a b) a.