#include <iostream>
#include <list>
using namespace std;
class A;
class B;
class C;
class D;
class A
{
};
class B : public A
{
friend class C;
friend class D;
private:
class BB
{
list<C> c;
list<D> d;
};
};
class C : public B::BB
{
};
class D : public B::BB
{
};
int main() {
// your code goes here
return 0;
}
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