code: def common_parent(self, node):
          
          p1 = self.parent_chain()
          p2 = node.parent_chain()
          for p in p1:
              if p in p2:
                  return p
          return None
docs: Return the common parent of two entities
  
          If the entities have no common parent, return None.
  
          Parameters
          ----------
          node : instance of Node
              The other node.
  
          Returns
          -------
          parent : instance of Node | None
              The parent.