????????
      +---------
VHPPU | RLQUHLQY
       -VHPPU
        =====
        YRMGWL
       -YVZWWN
        ======
         JMRGTQ
        -JVNTTQ
         ======
          YNUGGY
         -YNHHHM
          ======
          ??????

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