????????
      +---------
PMNPX | PQGNMLLT
       -PMNPX
        =====
         TRLRL
        -PMNPX
         =====
         MUNNPL
        -MGJRNM
         ======
          MUNHTT
         -MGJRNM
          ======
          ??????

base: 13
letters: FGHJLMNPQRTUX
difficulty: medium
missing: F