????????
      +---------
RXFFT | LFLLTPQM
       -EQUQU
        =====
         FFLQP
        -FXLFP
         =====
          ENENQ
         -WTWER
          =====
           MNPUM
          -XEXXF
           =====
           ?????

base: 13
letters: EFHLMNPQRTUWX
difficulty: medium
missing: H