template< class T >
  class A { 
    A<T>   
  public: 
  virtual T value() = 0;
  };
  
  class Int : public A<int> { //    Int  A<int>   
  public:
  Int(int n = 0) : _n(n) {}
  int value() { return _n; }
  private:
  int _n;
  };