func$ missing perms$[] .
   n = len perms$[1]
   len cnt[] n
   nn = 1
   for i = 2 to n - 1 : nn *= i
   for i to n
      for j to n : cnt[j] = 0
      for s$ in perms$[]
         cod = strcode substr s$ i 1 - 64
         cnt[cod] += 1
      .
      for j to n
         if cnt[j] <> nn : miss$ &= strchar (j + 64)
      .
   .
   return miss$
.
print missing [ "ABCD" "CABD" "ACDB" "DACB" "BCDA" "ACBD" "ADCB" "CDAB" "DABC" "BCAD" "CADB" "CDBA" "CBAD" "ABDC" "ADBC" "BDCA" "DCBA" "BACD" "BADC" "BDAC" "CBDA" "DBCA" "DCAB" ]