public NavigableSet<K> descendingSet() {
      return new NavigableKeySet(sortedMap().descendingMap());
    }

    @Override
    public Iterator<K> descendingIterator() {
      return descendingSet().iterator();
    }

    @Override
    public NavigableSet<K> headSet(@ParametricNullness K toElement) {
      return headSet(toElement, false);
    }

    @Override

    public NavigableSet<K> descendingSet() {
      return new NavigableKeySet(sortedMap().descendingMap());
    }

    @Override
    public Iterator<K> descendingIterator() {
      return descendingSet().iterator();
    }

    @Override
    public NavigableSet<K> headSet(@ParametricNullness K toElement) {
      return headSet(toElement, false);
    }

    @Override