????????
      +---------
RPQGG | QRNUVTWJ
       -TRXJJ
        =====
        NYRQNT
       -NZHMWV
        ======
         MQXWGW
        -MXJJYY
         ======
          NJNGPJ
         -NNMRGN
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging
missing: L