def input_signals(p):
    return list((p + p.lex_lead()).vars_as_monomial().variables())


def output_signal(p):
    return iter(p.lex_lead().variables()).next()


def rank(data):
    parents = dict()
    res = dict()
    for p in data:
    #        print p, output_signal(p)
        out = output_signal(p)
        parents.setdefault(out, [])
        for v in input_signals(p):
            parents.setdefault(v, []).append(out)

    def do_rank(v):
        if v in res:
            return res[v]
        my_res = res[v] = max([do_rank(p) + 1 for p in parents[v]] + [0])
        return my_res
    for v in parents.keys():
        do_rank(v)
    return res
