intros p; case p; simpl in |- *; auto with zarith.
intros p1; left; exists (Zpos p1); rewrite Zplus_comm;
simpl in |- *; auto.
intros p1; right; exists (Zpos p1); simpl in |- *; auto.
change
(forall p : positive,
{Odd (- Zpos p)} + {Even (- Zpos p)})
in |- *.
intros p; case p; auto with zarith.
intros p1; left; apply OddOpp; exists (Zpos p1);
rewrite Zplus_comm; simpl in |- *; auto.
intros p1; right; apply EvenOpp; exists (Zpos p1); simpl in |- *; auto.
Qed.

Theorem OddNEven : forall n : Z, Odd n -> ~ Even n.
intros n H1; red in |- *; intros H2; case H1; case H2; intros z1 Hz1 z2 Hz2.