<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set_exp(self, claim='exp', from_time=None, lifetime=None):
 """ Updates the expiration time of a token. """

        if from_time is None:
            from_time = self.current_time

        if lifetime is None:
            lifetime = self.lifetime

        self.payload[claim] = datetime_to_epoch(from_time + lifetime)