CustomMultimap(Map<K, Collection<V>> map, Supplier<? extends Collection<V>> factory) {
      super(map);
      this.factory = checkNotNull(factory);
    }

    @Override
    Set<K> createKeySet() {
      return createMaybeNavigableKeySet();
    }

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

    @Override

  }

  /** Returns an immutable sorted set of the keys in this map. */
  @Override
  public ImmutableSortedSet<K> keySet() {
    return keySet;
  }

  @Override
  ImmutableSet<K> createKeySet() {
    throw new AssertionError("should never be called");
  }

  /**
   * Returns an immutable collection of the values in this map, sorted by the ordering of the
   * corresponding keys.
   */