Insert an arg to the current expression.

def InsertArg(self, string="", **_):
    """Insert an arg to the current expression."""
    if self.verbose:
      logging.debug("Storing Argument %s", utils.SmartUnicode(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()