k = 0, n = 4 1000 xx00 x0x0 x00x

k = 1, n = 4 (k == n - 1) 1xxx // add x(-1)'s xx1x // clear previous x(-1)'s and then !!! GO AHEAD !!! xxxx x0xx

k = 2, n = 4

xxxx (no place where you can place an x then backtrack)

. . . succesfully placed it AND if (k == n-1) then PRINT THIS SOLUTION OUT and then CARRY ON!!!