@pick      @arr[@pick]

0 1 2  ==>  x x x . .
0 1 3  ==>  x x . x .
0 1 4  ==>  x x . . x
0 2 3  ==>  x . x x .
0 2 4  ==>  x . x . x
0 3 4  ==>  x . . x x
1 2 3  ==>  . x x x .
1 2 4  ==>  . x x . x
1 3 4  ==>  . x . x x
2 3 4  ==>  . . x x x