def arrange_lyrics(arrangement, song_data):
    """
    Returns the lyrics of a song arranged by the song data.

    :param arrangement: A list of strings describing the arrangement of the
                        lyrics. We do not do checking in this function, so the
                        type must be correct.
    :type arrangement: `list(str)`

    :param song_data: Song information, conforming to the model sepecification
                      in `datamodels.py`. One of the keys has to be `lyrics`.
    :type song_data: `dict`

    :returns: `arranged_lyrics`, the lyrics arranged according to the
              specified arrangement.
    :rtype: `str`
    """
    # Now, we allow the default arrangement to be set.
    arranged_lyrics = ""
    for a in arrangement:
        arranged_lyrics += song_data["lyrics"][a]
        arranged_lyrics += "\n\n"
    # print(arranged_lyrics)
    return arranged_lyrics