docs: def coords_to_num(coords):
      """
      >>> coords_to_num((1,1,1,1))
      1
      >>> coords_to_num((3,3,3,3))
      81
      >>> coords_to_num((1,1,1,3))
      3
      >>> coords_to_num((1,3,1,1))
      7
      >>> coords_to_num((3,3,3,1))
      79
      >>> coords_to_num((3,3,2,2))
      71
      >>> coords_to_num((2,1,1,1))
      28
      >>> coords_to_num((2,2,2,2))
      41
      """
      x1, y1, x2, y2 = coords
      row, col = (x1-1) * 3 + x2, (y1-1) * 3 + y2  # row,col on 9x9 board
      return (row-1) * 9 + col