segment.clear();
    }
  }

  @LazyInit @CheckForNull transient Set<K> keySet;

  @Override
  public Set<K> keySet() {
    Set<K> ks = keySet;
    return (ks != null) ? ks : (keySet = new KeySet());
  }

  @LazyInit @CheckForNull transient Collection<V> values;

  @Override
  public Collection<V> values() {
    Collection<V> vs = values;