hile (u, v) is the exclusive disjunction
of u and v, and ((u, v)) is the logical equivalence
of u and v.

o-----------------------------------------------------------o
|                                                           |
|                      (( ))  o  (r (c))                    |
|                            /|\                            |
|                           / | \                           |
|                          /  |  \                          |
|                         /   |   