.

                                      S
                      ­      q      q u       q
                         u          e a         u
                       e              l              e
                      a              e               a
                      l               r                 l
                      e           S     q             e
                      r            u    e             r
                       S           a     l             S
                        q           e   r            q  
                         u            S             u
                            e         q           e
                              a       u        a
                                 l     e     l
                                       a
                                       l
                                       e
                                       r