<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_user(self, nick, prefixes=None):
 """Add a user to our internal list of nicks."""

        if nick not in self._user_nicks:
            self._user_nicks.append(nick)

        self.prefixes[nick] = prefixes