python – 三重嵌套字典理解？

import pandas as pd
s = pd.Series(["hello go home bye bye","you can't always get","what you waaaaaaant","apple banana carrot munch 123"])

我想创建一个字典,其中单个字符作为键,其频率作为值.在集合的帮助下,为过去的单词创建这些词典很容易.计数器：

from collections import Counter
c = Counter(word for row in s for word in row.lower().split())

但是,我现在正在尝试存储单个字符,并且在三嵌套字典理解方面存在一些问题.这就是我所拥有的：

c = Counter((letter for letter in word) for word for row in s for word in row.lower().split())

这给了我一个语法错误.如何在一行中创建等效的以下for循环？

d = {}
for row in s:
    for word in row.lower().split():
        for letter in word:
            d[letter] += 1