'Post order walk op tree and produce postfix stack.
Arguments:
stack (list): user shall give empty list like `list()`, then it\'s
used recursively to construct the post-order stack.'

def traverse(self, stack):

    if isinstance(self[1], OpTreeNode):
        self[1].traverse(stack)
    elif (self[1] is not None):
        stack.append(self[1])
    if isinstance(self[2], OpTreeNode):
        self[2].traverse(stack)
    elif (self[2] is not None):
        stack.append(self[2])
    stack.append(self[0])
    return stack
