_                    -                     @     4 4        Diagram 2

automatically switches over to the other member of the pair.

_                    -                     @                Diagram 3


_                    -                     @                Diagram 4



_                    -                           X                        P    
        Diagram 5

_                    -                     @           l    Diagram 6


_                    -                           X                            
Diagram 7

_                    -                      Diagram 8

_                    -                      Diagram 9

_                    -                           X                             
      `                                   h    Diagram 10

_                    -                    Diagram 11

_                    -                    Diagram 12