def get_tw_refs(tw_refs_by_verse, book, chapter, verse):
    """ Retrurns a list of refs for the given book, chapter, verse, or
        empty list if no matches. """
    if book not in tw_refs_by_verse:
        return []
    if chapter not in tw_refs_by_verse[book]:
        return []
    if verse not in tw_refs_by_verse[book][chapter]:
        return []
    return tw_refs_by_verse[book][chapter][verse]