RFM
      +--------
UXLFP | MWFMTUF
       -UXLFP
        =====
        URXRLU
       -TXQLTW
        ======
         UYEEEF
        -URXYLF
         ======
          UPLYQ

base: 13
letters: EFJLMPQRTUWXY
difficulty: hard
missing: J