def pbox():
    """List for use in calculating pointing pair and triple."""
    pbox = {}
    for b in range(9):
        begin = [0, 3, 6, 27, 30, 33, 54, 57, 60][b]
        left = [0, 0, 0, 27, 27, 27, 54, 54, 54][b]
        top = [0, 3, 6, 0, 3, 6, 0, 3, 6][b]
        box = set([])
        for i in (0, 1, 2, 9, 10, 11, 18, 19, 20):
            box = box | set([begin + i])
        for j in range(3):
            line = set([])
            for i in range(9):
                line = line | set([left + j * 9 + i])
            pbox[b, j] = line - box
        for j in range(3, 6):
            line = set([])
            for i in range(9):
                line = line | set([top + j - 3 + i * 9])
            pbox[b, j] = line - box
    return pbox
