bit_nset
bit_nset(bs, 1, test_length - 2);
bit_nset(bs, 0, test_length - 2);
bit_nset(bs, i, test_length - 1);
bit_nset(bs, 0, test_length - 1);
bit_nset(bs, 0, test_length - 1 - i);
bit_nset(bs, 0, test_length - 1);
bit_nset(bs, 0, test_length - 1);
bit_nset(bs, i, i);