'Return a brief description of how to use this command, by
default from the attribute ``self.help``.'

def usage(self, subcommand):

    usage = (u'%%prog    %s    [options]    %s' % (subcommand, self.args))
    if self.help:
        return (u'%s\n\n%s' % (usage, self.help))
    else:
        return usage
