code: static boolean removeAllImpl(Multiset<?> self, Collection<?> elementsToRemove) {
      Collection<?> collection =
          (elementsToRemove instanceof Multiset)
              ? ((Multiset<?>) elementsToRemove).elementSet()
              : elementsToRemove;
  
      return self.elementSet().removeAll(collection);
    }
docs: An implementation of {@link Multiset#removeAll}.