code: def next_sibling(self):
          
          if self.parent is None:
              return None
  
          # Can't use index(); we need to test by identity
          for i, child in enumerate(self.parent.children):
              if child is self:
                  try:
                      return self.parent.children[i+1]
                  except IndexError:
                      return None
docs: The node immediately following the invocant in their parent's children
          list. If the invocant does not have a next sibling, it is None