(* Ugly tacty to resolve sign condition for R *)
Require Export RGroundTac.
(* Theorems to simplify the goal 0 ? x * y and x * y ? 0 where ? is < > <= >= *)
intros x y H; apply Rmult_le_pos; auto with real.
Qed.
intros x y H1 H2; replace (x * y)%R with (-x * -y)%R; auto with real; try ring.
apply Rmult_le_pos; auto with real.
Qed.
intros x H; rewrite <- (Ropp_involutive 0);  rewrite <- (Ropp_involutive x); auto with real.
apply Ropp_le_contravar; auto with real.
rewrite Ropp_0; auto with real.
Qed.
intros x y H1 H2; apply Rle_pos_neg; replace (- (x * y))%R with (x * (- y))%R; auto with real; try ring.
apply Rmult_le_pos; auto with real.
Qed.
intros x y H1 H2; apply Rle_pos_neg; replace (- (x * y))%R with (-x * y)%R; auto with real; try ring.
apply Rmult_le_pos; auto with real.
Qed.
intros; apply Rmult_lt_0_compat; auto with real.
Qed.
intros x y H1 H2; replace (x * y)%R with (-x * -y)%R; auto with real; try ring.
apply Rmult_lt_0_compat; auto with real.
Qed.
intros x H; rewrite <- (Ropp_involutive 0);  rewrite <- (Ropp_involutive x); auto with real.
apply Ropp_lt_contravar; auto with real.
rewrite Ropp_0; auto with real.
Qed.
intros x y H1 H2; apply Rlt_pos_neg; replace (- (x * y))%R with (x * (- y))%R; auto with real; try ring.
apply Rmult_lt_0_compat; auto.
replace 0%R with (-0)%R; auto with real.
Qed.

Theorem Rlt_sign_neg_pos: forall x y, (x < 0 -> 0 < y  -> x * y < 0)%R.