public interface IntMultiset {
   void add(int k);            // insert k into multiset--may produce duplicate versions of k
   void delete(int k);         // delete one instance of k--other instances of k may remain
   boolean contains(int k);   // does k occur in the multiset?
   boolean isEmpty();         // is the multiset empty?
   int size();                // total number of elements in the multiset (including duplicates)
}