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