*
 * <p>The multimap constructor takes a map that has a single entry for each distinct key. When you
 * insert a key-value pair with a key that isn't already in the multimap, {@code
 * AbstractMapBasedMultimap} calls {@link #createCollection()} to create the collection of values
 * for that key. The subclass should not call {@link #createCollection()} directly, and a new
 * instance should be created every time the method is called.

    return new CustomMultimap<>(map, factory);
  }

  private static final class CustomMultimap<K extends @Nullable Object, V extends @Nullable Object>
      extends AbstractMapBasedMultimap<K, V> {
    transient Supplier<? extends Collection<V>> factory;

    CustomMultimap(Map<K, Collection<V>> map, Supplier<? extends Collection<V>> factory) {
      super(map);