XHT
      +--------
ULWXG | LGVTUZL
       -WQLTY
        =====
        UJZGQZ
       -UVGHNU
        ======
          MRRVL
         -JZRHL
          =====
          RVGZP

base: 16
letters: GHJLMNPQRTUVWXYZ
difficulty: challenging
missing: P