intros a; case a; simpl in |- *; auto.
intros; apply (eqA_sym _ _ _ _ _ _ _ _ _ cs); apply invA_plusA with (1 := cs);
auto.
Qed.

Theorem zeroP_invTerm_zeroP : forall a : Term, zeroP a -> zeroP (invTerm a).
intros a; case a; simpl in |- *; auto.