docs: static <E> boolean addAllImpl(Multiset<E> self, Collection<? extends E> elements) {
      if (elements.isEmpty()) {
        return false;
      }
      if (elements instanceof Multiset) {
        Multiset<? extends E> that = cast(elements);
        for (Entry<? extends E> entry : that.entrySet()) {
          self.add(entry.getElement(), entry.getCount());
        }
      } else {
        Iterators.addAll(self, elements.iterator());
      }
      return true;
    }