(** Cancellations. *)
(** With the current convention, the following isn't always true when [c<0]: [-3*-1 / -2*-1 = 3/2 = 1] while [-3/-2 = 2] *)

Lemma div_mul_cancel_l : forall a b c, b~=0 -> 0<c ->
(c*a)/(c*b) == a/b.