????????
      +---------
PPNYY | JJYPTWVX
       -VZQVL
        =====
         WGGRW
        -WQGUR
         =====
          NWJZV
         -UULZZ
          =====
          RTPWZX
         -RHVRHU
          ======
          ??????

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: medium
hints: M << X
missing: M