@Override
    public Set<K> keySet() {
      return navigableKeySet();
    }

    @LazyInit @CheckForNull private transient NavigableSet<K> navigableKeySet;

    @Override
    public NavigableSet<K> navigableKeySet() {
      NavigableSet<K> result = navigableKeySet;
      return (result == null) ? navigableKeySet = new NavigableKeySet<>(this) : result;
    }

    @Override

    @Override
    public Set<K> keySet() {
      return navigableKeySet();
    }

    @LazyInit @CheckForNull private transient NavigableSet<K> navigableKeySet;

    @Override
    public NavigableSet<K> navigableKeySet() {
      NavigableSet<K> result = navigableKeySet;
      return (result == null) ? navigableKeySet = new NavigableKeySet<>(this) : result;
    }

    @Override