def add_section(self, section):
if not issubclass(section.__class__, SubSection):<EOL><INDENT>raise TypeError("<STR_LIT>" + str(section.__class__))<EOL><DEDENT>self.sections[section.name] = section<EOL>return section<EOL>
You can add section inside a Element, the section must be a
        subclass of SubSection. You can use this class to represent a tree.