code: protected Entry<K, V> standardLowerEntry(K key) {
      return headMap(key, false).lastEntry();
    }
docs: A sensible definition of {@link #lowerEntry} in terms of the {@code lastEntry()} of {@link
  #headMap(Object, boolean)}. If you override {@code headMap}, you may wish to override {@code
  lowerEntry} to forward to this implementation.