public static boolean retainOccurrences(
      Multiset<?> multisetToModify, Multiset<?> multisetToRetain) {
    return retainOccurrencesImpl(multisetToModify, multisetToRetain);
  }

  /** Delegate implementation which cares about the element type. */
  private static <E extends @Nullable Object> boolean retainOccurrencesImpl(
      Multiset<E> multisetToModify, Multiset<?> occurrencesToRetain) {
    checkNotNull(multisetToModify);