Multiset<E> multisetToModify, Multiset<?> occurrencesToRetain) {
    checkNotNull(multisetToModify);
    checkNotNull(occurrencesToRetain);
    // Avoiding ConcurrentModificationExceptions is tricky.
    Iterator<Entry<E>> entryIterator = multisetToModify.entrySet().iterator();
    boolean changed = false;
    while (entryIterator.hasNext()) {
      Entry<E> entry = entryIterator.next();