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