????????
      +---------
UPRPM | WRPRUFEX
       -UPRPM
        =====
        TMHMJF
       -TTFWHR
        ======
         XEWUWE
        -XHUTTE
         ======
          TTXULX
         -PRTJXT
          ======
          ??????

base: 13
letters: EFHJLMNPRTUWX
difficulty: hard
missing: N