????????
      +---------
QZQVL | QMXHTTLQ
       -QZQVL
        =====
         HJLRT
        -HWHTU
         =====
          JQMYL
         -HWHTU
          =====
          QQRLMQ
         -QWPJGR
          ======
          ??????

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