<div style="font-family: 'Times New Roman'; font-size: 16px;">Hello, I have made this : (define (string-insert s1 i) (if(> (string-length s1) 0) (string-append (substring s1 0 i) "-" (substring s1 i (string-length s1))))) But on the if I get a this message : if: expected a question and two answers, but found only 2 parts So I think that the if get sonehow messed up , What I try to do is this : When the lenght of the string is greater then 0 then take the first part of the string then put the - and the last part of the string. As last append the pieces to 1 piece. Without the if everything works great on non empty strings. Anyone a idea where I did make the wrong turn. Roelof </div>