????????
      +---------
GUHGU | UTUMTGMM
       -GUHGU
        =====
        XEYYEG
       -XMHXMH
        ======
         TYGTGM
        -TGHTGH
         ======
          XGHHMM
         -XMHXMH
          ======
          ??????

base: 12
letters: CEGHLMPRTUXY
missing: P L C R
difficulty: challenging
hints: P < R, C < X