back:2 0 [ 0, 7, 8, 9 ] [ 2, 1 ]
back:1 -1 [ 7, 8, 9 ] [ 3, 2 ]
lastfw 7,-1 [ 10, 3 ]
lastfw 8,-1 [ 11, 4 ]
lastfw 9,-1 [ 12, 5 ]
12 5