code: def add_child(self, child):
          
          if child.tag in self.contained_children:
              self.children.append(child)
          else:
              raise UNTLStructureException(
                  'Invalid child "%s" for parent "%s"' % (
                      child.tag,
                      self.tag
                  )
              )
docs: Add a child object to the current one.
  
          Checks the contained_children list to make sure that the object
          is allowable, and throws an exception if not.