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

Lemma Qdiv_Qdiv_Qmult_denominator: forall x y z, y<>Zero -> z<>Zero -> x/(y/z) = (x*z)/y.