code: def _merge_groups(self, backends_list):
          
          ret = {}
          for backends in backends_list:
              for b in backends:
                  if b not in ret:
                      ret[b] = set([])
                  for group in backends[b]:
                      ret[b].add(group)
          for b in ret:
              ret[b] = list(ret[b])
              ret[b].sort()
          return ret
docs: merge a list backends_groups