old_contents: #!/usr/bin/env python
  # vim: ai ts=4 sts=4 et sw=4
  
  
  class Backend(object):
      
      def log(self, level, message):
          self.router.log(level, message)
  
      def start(self):
          raise NotImplementedError
      
      def stop(self):
          raise NotImplementedError
      
      def send(self):
          raise NotImplementedError
      
      def receive(self):
          raise NotImplementedError
  
new_contents: #!/usr/bin/env python
  # vim: ai ts=4 sts=4 et sw=4
  
  class Backend(object):
      def __init__ (self, router):
          self.router = router
      
      def log(self, level, message):
          self.router.log(level, message)
  
      def start(self):
          raise NotImplementedError
      
      def stop(self):
          raise NotImplementedError
      
      def send(self):
          raise NotImplementedError
      
      def receive(self):
          raise NotImplementedError
  
subject: Add a constructor method for Backend