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 {