code: private static <E> boolean addAllImpl(Multiset<E> self, Multiset<? extends E> elements) {
      // It'd be nice if we could specialize for ImmutableMultiset here without also retaining
      // its code when it's not in scope...
      if (elements instanceof AbstractMapBasedMultiset) {
        return addAllImpl(self, (AbstractMapBasedMultiset<? extends E>) elements);
      } else if (elements.isEmpty()) {
        return false;
      } else {
        for (Multiset.Entry<? extends E> entry : elements.entrySet()) {
          self.add(entry.getElement(), entry.getCount());
        }
        return true;
      }
    }
docs: A specialization of {@code addAllImpl} for when {@code elements} is itself a Multiset.