class interval : public list<int> { 
  interval  
  public:
  interval(int x, int y) : _x(x), _y(y) { require( x <= y ); }
  bool empty() { return 0; }
  int head() { return _x; }
  list< int >* tail() {
    return (_x+1 <= _y)?
  	new interval(_x+1,_y):
  	new nil<int>;
    }
  
  bool operator==(list@lt;int>* m) { 
    return !m->empty() &&
    _x == m->head() && tail() == m->tail();
    }
  protected:
  int _x; int _y;
  };