def box():
    """Define the effective cells."""
    box = []
    for i in range(9):
        for j in range(9):
            b = [0] * 81
            for i2 in range(9):
                b[i2 * 9 + j] = 1
            for j2 in range(9):
                b[i * 9 + j2] = 1
            begin = i // 3 * 27 + j // 3 * 3
            for i2 in [0, 1, 2]:
                for j2 in [0, 1, 2]:
                    b[begin + i2 * 9 + j2] = 1
            b[i * 9 + j] = 0
            bb = []
            for i2 in range(81):
                if b[i2] == 1:
                    bb = bb + [i2]
            box = box + [bb]
    return box
