.
          
                                      B
                        ­      o    o  n     o
                           n       b  o        n
                         b           n             b
                        o            B               o
                       n             o                n
                      B              n                B
                      o          b       o            o
                       n         n      B            n
                        b          o   n            b
                         o            b             o
                           n          o           n
                             B        n        B
                                 o     B     o
                                        o
                                        n