def BFT(tree):
    """ Breadth-first treavsal on general RST tree

    :type tree: SpanNode instance
    :param tree: an general RST tree
    """
    queue = [tree]
    bft_nodelist = []
    while queue:
        node = queue.pop(0)
        bft_nodelist.append(node)
        queue += node.nodelist
    return bft_nodelist