See the documentation for __init__() to see an explanation of the 
        *backends argument.

def add_backends(self, *backends):
        """
        See the documentation for __init__() to see an explanation of the 
        *backends argument.
        """
        for backend in backends:
            full = self._expand_host(backend)
            self.backends[full] = 0
            self.task_counter[full] = 0