????????
      +---------
MUNFR | GFYNYNNG
       -MUNFR
        =====
        WMMFPN
       -PXHPFM
        ======
         MUPUPN
        -MHHQNM
         ======
          MTJRPG
         -MHHQNM
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: challenging
missing: L