????????
      +---------
XTNMP | NVRGNPJL
       -UWGXM
        =====
        ZVXUVP
       -ZRXNGR
        ======
          LLXVJ
         -JZQTH
          =====
          ZVJNNL
         -ZRXNGR
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: medium
missing: Y