code: def get_descendants_group_count(cls, parent=None):
          
          if parent is None:
              qset = cls.get_root_nodes()
          else:
              qset = parent.get_children()
          nodes = list(qset)
          for node in nodes:
              node.descendants_count = node.get_descendant_count()
          return nodes
docs: Helper for a very common case: get a group of siblings and the number
          of *descendants* (not only children) in every sibling.
  
          :param parent:
  
              The parent of the siblings to return. If no parent is given, the
              root nodes will be returned.
  
          :returns:
  
              A `list` (**NOT** a Queryset) of node objects with an extra
              attribute: `descendants_count`.