def set_comment(self, comment = None):
        """This method is used to the the comment.

        comment must be a string.

        """
        
        if comment is None or type(comment) is not str:
            raise KPError("Need a new image number")
        else:
            self.comment = comment
            self.last_mod = datetime.now().replace(microsecond=0)
            return True
This method is used to the the comment.

        comment must be a string.