def flow_cell_mode(info_reads):
    """Return flow cell sequencing mode."""
    res = ''
    if info_reads:
        read_lens = [
            a['num_cycles'] for a in info_reads if not a['is_indexed_read']]
        if len(read_lens) == 1:
            res = '1x{}'.format(read_lens[0])
        elif len(set(read_lens)) == 1:
            res = '2x{}'.format(read_lens[0])
        else:
            res = ' + '.join(map(str, read_lens))
        index_lens = [
            a['num_cycles'] for a in info_reads if a['is_indexed_read']]
        if index_lens:
            res += '/'
            if len(set(index_lens)) == 1:
                res += '{}x{}'.format(len(index_lens), index_lens[0])
            else:
                res += '+'.join(map(str, index_lens))
    return res or '?x?'