'Fetch truncated part of a message.
(typ, [data, ...]) = <instance>.partial(message_num, message_part, start, length)
\'data\' is tuple of message part envelope and data.'

def partial(self, message_num, message_part, start, length):

    name = 'PARTIAL'
    (typ, dat) = self._simple_command(name, message_num, message_part, start, length)
    return self._untagged_response(typ, dat, 'FETCH')
