def cut_neighbor_sequences(seq_s, flanking_i):
    """
    cut the flanking sequences
    :param seq_s: string, seq
    :param flanking_i: size of flanking seq
    :return: strings, cut (start), cut (the rest), cut (last)
    """
    assert type(seq_s) is str
    return seq_s[0:flanking_i], seq_s[flanking_i:-flanking_i], seq_s[-flanking_i:]