Hint Resolve Qlt_plus_pos_pos.

Lemma Qlt_dec_Qpos : forall x y, {~ Qlt (Qpos y) (Qpos x)} + {Qlt (Qpos y) (Qpos x)}.
intros x y; case Qpositive_le_dec with x y; intro H'; [left;intro H; inversion H; contradiction | right;unfold Qlt; constructor; trivial].