example answer..

gistfile1.py

codes = {}

def convert(tag):
	return codes.setdefault(tag, 1 << codes.length)


def desc(node):
	for child in node.childs:
		child_code = convert(child.tag)
		child.code = child_code | node.code
		if bool(child_code & node.code):
			print 'gotcha!!'
                desc(child)

root.code = convert(root.tag)
desc(root)