docs: def backends_to_mutate(self, namespace, stream):
      """
      Return all the backends enabled for writing for `stream`.
      """
      if namespace not in self.namespaces:
        raise NamespaceMissing('`{}` namespace is not configured'
                               .format(namespace))
      return self.prefix_confs[namespace][self.get_matching_prefix(namespace,
                                                                   stream)]