????????
      +---------
HRGYR | WYYWGJHL
       -WJHTQ
        =====
         RWXLJ
        -XXRQT
         =====
         HTLQHH
        -HHFTRL
         ======
          HRUYJL
         -HXXGUY
          ======
          ??????

base: 13
letters: FGHJLMQRTUWXY
difficulty: challenging
missing: M