docs: def update(self, members):
          """ Adds @members to the set
              @members: a :class:RedisSet object or #set
  
              -> #int the number of @members that were added to the set,
                  excluding pre-existing members
          """
          if isinstance(members, RedisSet):
              size = self.size
              return (self.unionstore(
                  self.key_prefix, members.key_prefix) - size)
          if self.serialized:
              members = list(map(self._dumps, members))
          if members:
              return self._client.sadd(self.key_prefix, *members)
          return 0