code: private static <E> boolean addAllImpl(
        Multiset<E> self, AbstractMapBasedMultiset<? extends E> elements) {
      if (elements.isEmpty()) {
        return false;
      }
      elements.addTo(self);
      return true;
    }
docs: A specialization of {@code addAllImpl} for when {@code elements} is an
  AbstractMapBasedMultiset.