Definition Zlog_sup (M : Z) :=
  match M with
  | Zpos p => log_sup p
  | _ => 0
  end.

Theorem approx_0_m2 : forall (y b : Zpls) (a : Z_ y) (x : Z_ (a * b + a)), x / y - 2 <= x / a * (a * b / y) / b <= x / y.