traverse-tree2.scm

(define (traverse-tree tree func)
  (define (spin cns prev)
    (let ((next (car cns)))
      (set-car! cns (cdr cns))
      (set-cdr! cns prev)
      next ))
  (let ((marker (gensym)))
    (let loop ((cns tree) (prev marker))
      (let ((next (spin cns prev)))
        (cond ((pair? next)
               (loop next cns) )
              ((not (eq? next marker))
               (or (null? next) (func next))
               (loop cns next) ))))) )

(define sample-list '((3 (5 (7)) 11)))

(let ((tree (list-copy sample-list)))
  (traverse-tree tree print)
  (print tree) )