????????
      +---------
LUTJG | FRQLWXGU
       -LUTJG
        =====
        JWUJPX
       -JRRNXF
        ======
         YHFTRG
        -YRQHJU
         ======
          YXHLJU
         -YPXWLN
          ======
          ??????

base: 15
letters: FGHJLMNPQRTUWXY
difficulty: hard
missing: M