_0 and x_ is a 3D version of Tic-Tac-Toe or Noughts and Crosses, where you
have to make a row of X’s in a vertical, horizontal or diagonal line before
the computer makes a row. Instead of making a row off three on a 3 x 3 grid
you now have to make a row off four on a 4 x 4 grid on four boards so as well
as watching for a line on one board, you have to watch out for a row that
appears in the same place on all four boards.