code: def _gen_back(self):
          '''
          Return a list of loaded roster backends
          '''
          back = set()
          if self.backends:
              for backend in self.backends:
                  fun = '{0}.targets'.format(backend)
                  if fun in self.rosters:
                      back.add(backend)
              return back
          return sorted(backf _gen_back(self):
          '''
          Return a list of loaded roster backends
          '''
          back = set()
          if self.backends:
              for backend in self.backends:
                  fun = '{0}.targets'.format(backend)
                  if fun in self.rosters:
                      back.add(backend)
              return back
          return sorted(back)
docs: Return a list of loaded roster backends