def InsertArg(self, string='', **unused_kwargs):
    """Insert an argument to the current expression."""
    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
Insert an argument to the current expression.