code: protected Entry<K, V> standardCeilingEntry(K key) {
      return tailMap(key, true).firstEntry();
    }
docs: A sensible definition of {@link #ceilingEntry} in terms of the {@code firstEntry()} of {@link
  #tailMap(Object, boolean)}. If you override {@code tailMap}, you may wish to override {@code
  ceilingEntry} to forward to this implementation.