do_swaps: 0 .. 6
[1 b c d e f a 2 3] 0 <=> 6
[1 2 c d e f a b 3] 1 <=> 7
[1 2 3 d e f a b c] 2 <=> 8
[1 2 3 a e f d b c] 3 <=> 6
[1 2 3 a b f d e c] 4 <=> 7
[1 2 3 a b c d e f] 5 <=> 8
do_swaps: 6 .. 9
 6 swaps => 1 2 3 a b c d e f