function find(str) {
  for (y = 2; y <= Bottom; y++) {
    if ([3,y] == str) {
      return y;
    }
  }
  return 0;
}

for (y = 2; y <= Bottom; y++) {
  yy = find([1,y]);
  if (yy > 0) {
    [2,y] = [4,yy];
  }
}