code: def get_prev_sibling(self):
          
          siblings = self.get_siblings()
          ids = [obj.pk for obj in siblings]
          if self.pk in ids:
              idx = ids.index(self.pk)
              if idx > 0:
                  return siblings[idx - 1]
docs: :returns:
  
              The previous node's sibling, or None if it was the leftmost
              sibling.