return (result == null) ? complement = new Complement() : result;
  }

  @VisibleForTesting
  static final class RangesByUpperBound<C extends Comparable<?>>
      extends AbstractNavigableMap<Cut<C>, Range<C>> {
    private final NavigableMap<Cut<C>, Range<C>> rangesByLowerBound;

    /**
     * upperBoundWindow represents the headMap/subMap/tailMap view of the entire "ranges by upper