class A { 
   ancestor  
  public:
  A() { n = 0; }
  void add( int i ) { n = n + i; }
  virtual int val() { return n; }
  protected: 
 private would deny access to D
  int n;
  };
  
  class D : public A { 
   descendant  
  public:
  D() : A() { }
  int val() { return n $\%$ 2; }
  };