????????
      +---------
NQGML | FFTHMHPY
       -GYMLF
        =====
         UPTNH
        -URWTL
         =====
          HWFTP
         -HYJUR
          =====
           GPMPY
          -NQGML
           =====
           ?????

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