def postorder_DFT(tree, nodelist):
    """ Post order traversal on binary RST tree

    :type tree: SpanNode instance
    :param tree: an binary RST tree

    :type nodelist: list
    :param nodelist: list of node in post order
    """
    if tree.lnode is not None:
        postorder_DFT(tree.lnode, nodelist)
    if tree.rnode is not None:
        postorder_DFT(tree.rnode, nodelist)
    nodelist.append(tree)
    return nodelist