;6.)

(define (prepare x)
  (cond ((number? x) (number->string x))
        ((char? x) (string x))))

(define (parse-prefix T)
  (let ((string-t (tree-map T prepare)))
    (if (null? string-t) ""
        (string-append (value string-t)
                       (parse-prefix (left string-t))
                       (parse-prefix (right string-t))))))

(parse-prefix example)