program SymmetricDifference;
type
charSet = set of Char;
var
s1, s2, s3: charSet;
ch: char;
begin
s1 := ['a', 'b', 'c', 'd'];
s2 := ['c', 'd', 'e', 'f'];
s3 := s1 >< s2;
for ch in s3 do
write(ch, ' ');
writeLn;
end.