code: def InsertArg(self, string='', **unused_kwargs):
      
      logging.debug('Storing Argument {0:s}'.format(string))
  
      # This expression is complete
      if self.current_expression.AddArg(string):
        self.stack.append(self.current_expression)
        self.current_expression = self.expression_cls()
        return self.PopState()
      return None
docs: Insert an argument to the current expression.