#include <iostream>
#include <functional>
#include <memory>
 
struct ACancelable {};
struct ACancelableToken {};
 
template<typename T, typename U>
class ANotifier : public ACancelable
{
public:
  using NotificationFn = std::function<void(T&)>;
  virtual std::weak_ptr<ACancelableToken> RegisterNotification( U msgType, NotificationFn fn ) = 0;
  virtual void Notify( U msgType, T& value ) = 0;
};
 
template<typename T, typename U>
class ASingleNotifier : public ANotifier<T, U>
{
  using NotificationFn = typename ANotifier<T,U>::NotificationFn;
  virtual std::weak_ptr<ACancelableToken> RegisterNotification( U msgType, NotificationFn fn );
};
 
int main() {
	// your code goes here
	return 0;
}

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