Hi Wagner,
my main-problem was, that I didn't know SetOf([]).
These are my very first steps with TScripter - and probably I did not read the manual close enough.
Anyhow - I finally got it work. Since I'm a lazy guy, I took this one...
aString := StringReplace(aString, 'a', 'b', SetOf([0, 1]));
Thanks a lot for your support.
Regards, Tom