.
                                       c
                              a     a b      a
                            b       a n        b
                           a          a            a
                          n        c     a         n
                          a        b      a         a
                          c         n     a          c
                           a        c     a          a
                            b        b   a         b
                             a          n           a
                              n                     n
                                         a