#include <iostream>
#include <string>

using namespace std;

class A {
public:
  int a;
  A(int a): a(a){}
};

class D : public A, public A {  // error: duplicate base type A invalid
public:
  int d;
  D(): d(9){}  
};


int f(D &x){
  // problem: how should we select one or the other A branch?
  cout << x.d << endl;     
}

int main(){
  D d;
  f(d);
}