'Modify the object at dn using the attribute list.
:param dn: a dn
:param attrs: a list of tuples in the following form::
([MOD_ADD | MOD_DELETE | MOD_REPACE], attribute, value)'

def modify_s(self, dn, attrs):

    if server_fail:
        raise SERVER_DOWN()
    store = Store.instance()
    key = ('%s%s' % (self.__prefix, dn))
    for (cmd, k, v) in attrs:
        values = _from_json(store.hget(key, k))
        if (cmd == MOD_ADD):
            values.append(v)
        elif (cmd == MOD_REPLACE):
            values = [v]
        else:
            values.remove(v)
        store.hset(key, k, _to_json(values))
