.

                                       P
                               l     l   a     l
                          a         n  C         a
                        n             P              n
                       C               l                C
                      P                a      ­           P
                      l                 n                  l
                      a             P       l             a
                       n            a      n            n
                        C            C    P            C
       ­                   P             l               P
                             l           a            l
                                a       n        a
                                     n  C   n