docs: def to_xml(self):
          '''
          Returns a DOM representation of the group.
          @return: Element
          '''
          if not len(self.lines):
              raise GroupError("A group must at least have one line.")
  
          doc = Document()
          root = doc.createElement("group")
          super(Group, self).to_xml(root)
          self._create_text_node(root, "name", self.name, True)
          self._create_text_node(root, "description", self.description, True)
  
          lines = doc.createElement("lines")
          root.appendChild(lines)
          for line in self.__lines:
              if not issubclass(line.__class__, Line):
                  raise GroupError('line of type %s is not an instance ' \
                                       'or a subclass of %s' %
                                       (line.__class__.__name__, Line.__name__))
              lines.appendChild(line.to_xml())
  
          return root