<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def register_calendar_alias(self, alias, real_name, force=False):
 """ Register an alias for a calendar. This is useful when multiple exchanges should share a calendar, or when there are multiple ways to refer to the same exchange. After calling ``register_alias('alias', 'real_name')``, subsequent calls to ``get_calendar('alias')`` will return the same result as ``get_calendar('real_name')``. Parameters alias : str The name to be used to refer to a calendar. real_name : str The canonical name of the registered calendar. force : bool, optional If True, old calendars will be overwritten on a name collision. If False, name collisions will raise an exception. Default is False. """

        if force:
            self.deregister_calendar(alias)

        if self.has_calendar(alias):
            raise CalendarNameCollision(calendar_name=alias)

        self._aliases[alias] = real_name

        # Ensure that the new alias doesn't create a cycle, and back it out if
        # we did.
        try:
            self.resolve_alias(alias)
        except CyclicCalendarAlias:
            del self._aliases[alias]
            raise