n =   1        2         3         4           5
      R        RW       RWR        RWRW        RWRWR
      W        WR       WRW        WRWR        WRWRW
                        RBW        RBWR        RBWRW
                        WBR        WBRW        WBRWR
                                   RWBR        RWBRW
                                   WRBW        WRBWR
                                               RWRBW
                                               WRWBR
                                               RBWBR
                                               WBRBW

Each column can be derived from:
1. the column on its left + R or W
and
2. the column on its left's left + BR or BW