.

                                       B
                             D       D  S      D
                          S           M B          S
                         M            B              M
                        B          ­    D                B
                       D               S                  D
                      S               M                   S
                     M            B      D             M
                     B            S         M            B
                      D            B       D            D
                        S             S    M           S
                          M            B            M
                              B         D          B
                                  D     S      D
                                      S  M  S
          ­                                B
                               ­           D
                                           S
                                          M