.
                                    P
                         ­  i       i  e         i
                        e         c e            e
                       c            P              c
                       e         i     e            e
                       P         c     e           P
                       i          P     i            i
                        e         e   c           e
                         c           e            c
                           P         P         P
                                      e
                       ­               a
                                      c
                                      e