// https://www.youtube.com/watch?v=nLSm3Haxz0I 

class EventSource
{
  void add(Listener& listener);
  void remove(Listener& listener);
public:
  [[nodiscard]]
  Subscription subscribe(Listener& listener) {
    return Subscription(*this, listener); }
};

class EventSource::Subscription
{
  Subscription(EventSource& source, Listener& listener)
    : source_(source), listener_(listener) 
    { source_.add_listener(listener_); }

  ~Subscription()
  { source_.remove(listener_);}
  
  // special function members
};