????????
      +---------
JPXYV | NWWXMGMN
       -JPXYV
        =====
        YHLLJG
       -YLUXHW
        ======
         JTQXJM
        -GQRYPQ
         ======
          GYURQN
         -YQYXQY
          ======
          ??????

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