code: def prev_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:
                  if i == 0:
                      return None
                  return self.parent.children[i-1]
docs: The node immediately preceding the invocant in their parent's children
          list. If the invocant does not have a previous sibling, it is None.