????????
      +---------
GMGNL | HVJYLQPX
       -VXPYG
        =====
         LWUWQ
        -LVVMX
         =====
          YNLJP
         -GMGNL
          =====
           PXWHX
          -TLZZP
           =====
           ?????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: hard
missing: R