def get_first_df(corpus):
    """
    Get the first document in a corpus, so that we can
    check what columns it has
    """
    from corpkit.corpus import Corpus
    if not isinstance(corpus, Corpus):
        corpus = Corpus(corpus, print_info=False)
    if corpus.subcorpora:
        return corpus.subcorpora[0].files[0].document
    elif corpus.files:
        return corpus.files[0].document
    else:
        return corpus.document
