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