prop_differenceVersionRanges :: VersionRange -> VersionRange -> Version -> Bool
prop_differenceVersionRanges vr1 vr2 v' =
     withinRange v' (differenceVersionRanges vr1 vr2)
  == (withinRange v' vr1 && not (withinRange v' vr2))
prop_differenceVersionRanges :: VersionRange -> VersionRange -> Version -> Bool
prop_differenceVersionRanges vr1 vr2 v' =
     withinRange v' (differenceVersionRanges vr1 vr2)
  == (withinRange v' vr1 && not (withinRange v' vr2))
prop_differenceVersionRanges vr1 vr2 v' =
     withinRange v' (differenceVersionRanges vr1 vr2)
  == (withinRange v' vr1 && not (withinRange v' vr2))