def sub_dir_by_split(d):
    """ build out the split portion of the results directory structure.
    :param dict d: A dictionary holding BIDS terms for path-building
    """
    return "_".join([
        '-'.join(['parby', d['parby'], ]),
        '-'.join(['splby', d['splby'], ]),
        '-'.join(['batch', d['batch'], ]),
    ])