corpuses = {
    "The Wasteland": "I will show you fear in a handful of dust. In the mountains there you feel free. A crowd flowed over London Bridge.",
    "The Road Not Taken": "Two roads diverged in a yellow wood. I kept the first for another day. I doubted if I should ever come back.",
    "Trees": "I think that I shall never see a poem lovely as a tree.  A tree that looks at God all day and lifts her leafy arms to pray."
}

def digest(corpus):
    """digests a single piece of corpus into a dictionary of the form {word: {sentence: frequency}}"""
    result = {}
    for sentence in corpus.split("."):
        words = [word.lower().strip() for word in sentence.strip().split()]
        for word in words:
            frequency = words.count(word)
            result.setdefault(word, {})[sentence] = frequency
    return result

def combine_corpuses(corpuses):
    result = {}
    all_words = {word for corpus in corpuses for word in corpus.keys()}
    for word in all_words:
        combined_d = {}
        for corpus in corpuses:
            combined_d.update(corpus.get(word, {}))
        result[word] = combined_d
    return result

digested_corpuses = [digest(corpus) for title, corpus in corpuses.items()]
single_digest = combine_corpuses(digested_corpuses)
print(single_digest)