Berkeley, 1990.

We begin with a pair of 2-adic relations G, H c X x Y.

o-------------------------------------------------o
|                                                 |
|        o                       o                |
|        |\                      |\               |
|        | \                     | \              |
|        |  \                    |  \             |
|        |   \                   |   \            |
|        |    \                  |    \           |
|        |     \       