* "https://github.com/google/guava/wiki/ImmutableCollectionsExplained">immutable collections</a>.
 *
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible
public abstract class ImmutableMultimap<K, V> extends BaseImmutableMultimap<K, V>
    implements Serializable {

  /**
   * Returns an empty multimap.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */