def volume_fn(cnt):
    """
    Construct filename for 'volume' metadata file.

    Args:
        cnt (int): Number of the MODS record.

    Returns:
        str: Filename in format ``mods_volume.xml`` or ``mods_volume_cnt.xml``.
    """
    return 'mods_volume%s.xml' % ('_%d' if cnt > 0 else '')
