text: #include <observer.h>
  #include <iostream>
  
  void hello_world()
  {
      std::cout << "Hello World!" << std::endl;
  }
  
  int main( int /* argc */, char * /* argv */[] )
  {
      pg::subject<> s;
  
      auto connection = pg::connect( s, hello_world );
  
      s.notify();
  
      return 0;
  }
  