code: public static <A, B> Multiset<B> mutableTransformedCopy(Multiset<A> ms,
        Function<A, B> func) {
      final Multiset<B> ret = HashMultiset.create();
  
      for (final Multiset.Entry<A> entry : ms.entrySet()) {
        final B transformedElement = func.apply(entry.getElement());
        ret.add(transformedElement, entry.getCount());
      }
  
      return ret;
    }
docs: Same as transformedCopy, except the returned Multiset is mutable.