async def extend(self, additional_time):
        """
        Adds more time to an already acquired lock.

        ``additional_time`` can be specified as an integer or a float, both
        representing the number of seconds to add.
        """
        if self.local.token is None:
            raise LockError("Cannot extend an unlocked lock")
        if self.timeout is None:
            raise LockError("Cannot extend a lock with no timeout")
        return await self.do_extend(additional_time)