def enum(word_sentences, tag_sentences):
    """
    enumerate words, chars and tags for
    constructing vocabularies.
    """

    words = sorted(list(set(sum(word_sentences, []))))
    chars = sorted(list(set(sum([list(word) for word in words], []))))
    tags = sorted(list(set(sum(tag_sentences, []))))

    return words, chars, tags