code: def gets(self, conn, key, default=None):
          
          values, cas_tokens = yield from self._multi_get(
              conn, key, with_cas=True)
          return values.get(key, default), cas_tokens.get(key)
docs: Gets a single value from the server together with the cas token.
  
          :param key: ``bytes``, is the key for the item being fetched
          :param default: default value if there is no value.
          :return: ``bytes``, ``bytes tuple with the value and the cas