 as the additive identity:
   anything unioned with an empty value is itself. Anything minus an empty
   value is itself. An empty multirange has exactly the same points as an empty
   range. Every range contains the empty range. Every multirange contains as many
   empty ranges as you like.
  </p><p>
   The range union and difference operators will fail if the resulting range would
   need to contain two disjoint sub-ranges, as such a range cannot be
   represented. There are separate operators for union a