intros x y z H H0 H1 H2 H3.
replace y with (Qpositive_sub z (Qpositive_sub z y)).
replace x with (Qpositive_sub z (Qpositive_sub z x)).
apply Qpositive_le_sub_l.
apply sym_not_equal; apply Qpositive_sub_diff; auto.
apply sym_not_equal; apply Qpositive_sub_diff; auto.
apply Qpositive_sub_le; auto.
apply Qpositive_sub_le; auto.
auto.
rewrite Qpositive_sub_sub_assoc; auto.
rewrite Qpositive_plus_sym; rewrite Qpositive_sub_correct; auto.
apply Qpositive_sub_diff.
auto.
auto.
apply Qpositive_sub_le; auto.
rewrite Qpositive_sub_sub_assoc; auto.
rewrite Qpositive_plus_sym; rewrite Qpositive_sub_correct; auto.
apply Qpositive_sub_diff.
auto.
auto.
apply Qpositive_sub_le; auto.
Qed.

Theorem Qgt_plus : forall x y z : Q, Qgt x y -> Qgt (Qplus z x) (Qplus z y).
intros x y z H; elim H.