'Set custom value
@param key: the index for the custom field (between 1-5)
@type key: int
@param value: the value to be stored
@type value: str
@return: if successful, value will be returned
@rtype: str'

def set_custom(self, key, value):

    self._assert_custom_key_valid(key)
    m = rtorrent.rpc.Multicall(self)
    self.multicall_add(m, 'd.set_custom{0}'.format(key), value)
    return m.call()[(-1)]
