V perms = [‘ABCD’, ‘CABD’, ‘ACDB’, ‘DACB’, ‘BCDA’, ‘ACBD’, ‘ADCB’, ‘CDAB’,
           ‘DABC’, ‘BCAD’, ‘CADB’, ‘CDBA’, ‘CBAD’, ‘ABDC’, ‘ADBC’, ‘BDCA’,
           ‘DCBA’, ‘BACD’, ‘BADC’, ‘BDAC’, ‘CBDA’, ‘DBCA’, ‘DCAB’]

V missing = ‘’
L(i) 4
   V cnt = [0] * 4
   L(j) 0 .< perms.len
      cnt[perms[j][i].code - ‘A’.code]++
   L(j) 4
      I cnt[j] != factorial(4-1)
         missing ‘’= Char(code' ‘A’.code + j)
         L.break

print(missing)