Python - juntar árboles anytree

#-*- coding: utf -8 -*-

from heapq import heapify, heappush, heappop

from itertools import count

def huffman(seq, frq):

    num = count()

    trees = list(zip(frq, num, seq)) # num es para asegurar un orden válido}

    heapify(trees) # un mínimo basado en la frecuencia 

    while len(trees) > 1: # hasta que todos se combinen

        fa, _, a = heappop(trees) #conseguir dos arboles mas pequeños

        fb, _, b = heappop(trees)

        n = next(num)

        heappush(trees, (fa+fb, n, [a, b])) # combina y agrega

    return trees[0][-1]

seq = "abcdefghi"

frq = [4,5, 6, 9, 11, 12, 15, 16, 20]

print(huffman(seq, frq))