'Request roster from server if it were not yet requested
(or if the \'force\' argument is set).'

def Request(self, force=0):

    if (self.set is None):
        self.set = 0
    elif (not force):
        return
    self._owner.send(Iq('get', NS_ROSTER))
    self.DEBUG('Roster    requested    from    server', 'start')
