<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def seen(self, event, nickname):
 """ Shows the amount of time since the given nickname was last seen in the channel. """

        try:
            self.joined[nickname]
        except KeyError:
            pass
        else:
            if nickname == self.get_nickname(event):
                prefix = "you are"
            else:
                prefix = "%s is" % nickname
            return "%s here right now" % prefix
        try:
            seen = self.timesince(self.quit[nickname])
        except KeyError:
            return "%s has never been seen" % nickname
        else:
            return "%s was last seen %s ago" % (nickname,  seen)