????????
      +---------
GFMRP | MYHQHLMQ
       -GFMRP
        =====
        PFGHWL
       -PJUGJN
        ======
         NQQWMM
        -NXRJJL
         ======
          FHWGHQ
         -FYHWRU
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: medium
missing: T