$ include "seed7_05.s7i";

const func string: missingPermutation (in array string: perms) is func
  result
    var string: missing is "";
  local
    var integer: pos is 0;
    var set of char: chSet is (set of char).EMPTY_SET;
    var string: permutation is "";
    var char: ch is ' ';
  begin
    if length(perms) <> 0 then
      for key pos range perms[1] do
        chSet := (set of char).EMPTY_SET;
        for permutation range perms do
          ch := permutation[pos];
          if ch in chSet then
            excl(chSet, ch);
          else
            incl(chSet, ch);
          end if;
        end for;
        missing &:= min(chSet);
      end for;
    end if;
  end func;

const proc: main is func
  begin
    writeln(missingPermutation([] ("ABCD", "CABD", "ACDB", "DACB", "BCDA", "ACBD",
           "ADCB", "CDAB", "DABC", "BCAD", "CADB", "CDBA", "CBAD", "ABDC", "ADBC",
           "BDCA", "DCBA", "BACD", "BADC", "BDAC", "CBDA", "DBCA", "DCAB")));
  end func;