Definition Qmult_resp_Qlt_pos_l:= Qlt_Qmult_cancel_l: forall t x y : Q, Zero < t -> t * x < t * y -> x < y.

Lemma Qmult_mult_Qle_Qone_Qopp_Qone: forall q, -Qone<=q -> q<= Qone -> q*q <= Qone.