<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def netengine(self):
 """ access netengine instance """

        # return None if no backend chosen yet
        if not self.backend:
            return None

        # init instance of the netengine backend if not already done
        if not self.__netengine:
            NetengineBackend = self.backend_class
            arguments = self._build_netengine_arguments()

            self.__netengine = NetengineBackend(**arguments)

        # return netengine instance
        return self.__netengine