checkArgument(
          lowerBound == null || upperBound == null || compare(lowerBound, upperBound) <= 0);
    }

    @Override
    public SortedSet<C> keySet() {
      return new Maps.SortedKeySet<>(this);
    }

    @Override
    public Comparator<? super C> comparator() {
      return columnComparator();
    }

    int compare(Object a, Object b) {
      // pretend we can compare anything

      return new SortedKeySet<>(map().subMap(fromElement, toElement));
    }

    @Override
    public SortedSet<K> headSet(@ParametricNullness K toElement) {
      return new SortedKeySet<>(map().headMap(toElement));
    }

    @Override
    public SortedSet<K> tailSet(@ParametricNullness K fromElement) {
      return new SortedKeySet<>(map().tailMap(fromElement));
    }

      return new SortedKeySet<>(map().subMap(fromElement, toElement));
    }

    @Override
    public SortedSet<K> headSet(@ParametricNullness K toElement) {
      return new SortedKeySet<>(map().headMap(toElement));
    }

    @Override
    public SortedSet<K> tailSet(@ParametricNullness K fromElement) {
      return new SortedKeySet<>(map().tailMap(fromElement));
    }