case y; case x; try (intros; simpl in Htrue; discriminate; fail).
(* Focus 1 *)
auto.
(* Focus 2 *)
intros x' y'; simpl in |- *; case (Qpositive_eq_dec x' y').
intros H; rewrite H; auto.
intros; discriminate.
(* Focus 3 *)
intros x' y'; simpl in |- *; case (Qpositive_eq_dec x' y').
intros H; rewrite H; auto.
intros; discriminate.
Qed.

Theorem not_one_zero : Qpos One <> Zero.
discriminate.