Multimap<? extends K, ? extends V> multimap) {
    this(keyComparator, valueComparator);
    putAll(multimap);
  }

  @Override
  Map<K, Collection<V>> createAsMap() {
    return createMaybeNavigableAsMap();
  }

  /**
   * {@inheritDoc}
   *
   * <p>Creates an empty {@code TreeSet} for a collection of values for one key.
   *