parameters: (self)
docstring: Return a post-order iterator for the tree.
  
          This must be implemented by the concrete subclass.
docstring_summary: Return a post-order iterator for the tree.
docstring_tokens: Return a post - order iterator for the tree .
function: def post_order(self):
          """
          Return a post-order iterator for the tree.
  
          This must be implemented by the concrete subclass.
          """
          raise NotImplementedError
function_tokens: def post_order ( self ) : raise NotImplementedError