You can add section inside a Element, the section must be a
        subclass of SubSection. You can use this class to represent a tree.

def add_section(self, section):
        """You can add section inside a Element, the section must be a
        subclass of SubSection. You can use this class to represent a tree.
        """

        if not issubclass(section.__class__, SubSection):
            raise TypeError("Argument should be a subclass of SubSection, \
                             not :" + str(section.__class__))
        self.sections[section.name] = section
        return section