| · | a | b | x1 | x2 | y1 | y2 |
| a | a | 0 | 0 | 0 | a | a |
| b | 0 | 0 | 0 | 0 | x2 | x1 |
| x1 | 0 | 0 | 0 | 0 | 0 | x1 |
| x2 | 0 | 0 | 0 | 0 | x2 | 0 |
| y1 | a | x2 | 0 | x2 | y1 | a |
| y2 | a | x1 | x1 | 0 | a | y2 |