+---+       +---+
                         | X |       | Y |
                         +---+       +---+
                           |           |
                           ---->XOR<----
                         --------|
                         |      \ /
                         |     +---+
                         |     | R |
                         |     +---+
                         |       |
                         |      \ /
                         |   P rowwise
                         |       |
                         |      \ /
                         |     +---+
                         |     | Q |
                         |     +---+
                         |       |
                         |      \ /
                         |  P columnwise
                         |       |
                         |      \ /
                         |     +---+
                         |     | Z |
                         |     +---+
                         |       |
                         |      \ /
                         ------>XOR
                                 |
                                \ /