'Insert an arg to the current expression.'

def InsertArg(self, string='', **_):

    if self.verbose:
        logging.debug('Storing    Argument    %s', utils.SmartUnicode(string))
    if self.current_expression.AddArg(string):
        self.stack.append(self.current_expression)
        self.current_expression = self.expression_cls()
        return self.PopState()
