code: public void subtract(Object o) {
      int[] count = (int[]) cset.get(o);  
      if ( count != null ) { 
        count[0]--;
        if (count[0] <= 0) {
          cset.remove(o);
        }
      }
    }
docs: Reduces the count associated with this object by 1.  If this causes the count
  to become 0, then the object is removed form the set.
  @param o The object whose count is being reduced.