* populate a map to be passed to {@link Multimaps#forMap(Map)}. All tests should run against the
   * result of {@link #build()}.
   */
  private static final class PopulatableMapAsMultimap<K, V> extends ForwardingMultimap<K, V> {
    final Map<K, V> map;
    final SetMultimap<K, V> unusableDelegate;

    static <K, V> PopulatableMapAsMultimap<K, V> create() {
      return new PopulatableMapAsMultimap<>();
    }