intros w w'.
elim (interp_non_zero w); intros p Hex; elim Hex; intros q Heq; clear Hex.
elim (interp_non_zero w'); intros p' Hex; elim Hex; intros q' Heq'; clear Hex.
unfold Qpositive_sub, Qpositive_plus in |- *.
rewrite Heq; rewrite Heq'.
elim
(construct_correct2' (S p * S q' + S p' * S q + S q * S q')
(S p * S q' + S p' * S q) (S q * S q')).
intros d;
elim
(interp_non_zero
(Qpositive_c (S p * S q' + S p' * S q) (S q * S q')
(S p * S q' + S p' * S q + S q * S q'))); intros p3 Hex;
elim Hex; intros q3 Heq3; rewrite Heq3; intros (Heq1, Heq2);
clear Hex.
rewrite <- (construct_correct _ _ _ (S p + S q) Heq).
apply construct_correct4'; try (simpl in |- *; auto with arith; fail).
unfold fst, snd in Heq1, Heq2.
apply lt_le_S.
apply lt_mult_conv with d.
rewrite <- mult_n_O.
unfold lt in |- *.
rewrite (mult_comm (S d)).
rewrite mult_minus_distr_r.
rewrite (mult_comm (S p3)); repeat rewrite <- mult_assoc.
rewrite <- Heq2; rewrite <- Heq1.
rewrite (mult_comm (S q')); rewrite mult_plus_distr_r.
rewrite plus_comm.
rewrite <- mult_assoc.
rewrite minus_plus.
simpl in |- *; auto with arith.
apply plus_le_compat_r.
apply le_plus_trans.
apply le_minus.
unfold fst, snd in Heq1, Heq2.
apply mult_reg_l with d.
rewrite mult_minus_distr_r; rewrite (mult_comm (S d));
repeat
(rewrite (mult_comm (S q3)) || rewrite (mult_comm (S p3));
repeat rewrite <- mult_assoc); repeat rewrite mult_minus_distr_r.
repeat rewrite (mult_comm (S d)); repeat rewrite <- mult_assoc.
rewrite <- Heq1; rewrite <- Heq2.
rewrite (mult_comm (S q)); rewrite (mult_comm (S q'));
repeat rewrite mult_plus_distr_r.
repeat rewrite mult_assoc.
rewrite plus_comm; apply minus_plus.
auto.
simpl in |- *; auto with arith.
simpl in |- *; auto with arith.
auto.
Qed.

Theorem le_minus_left :
forall a b c : nat, b <= a -> c <= a -> c <= b -> a - b <= a - c.