indexed_tree.scm

(define (make-itree)
  (list (list) (list)))
  
(define (add-index! itree root-dest)
  (define (add-index-rest! index)
    (cond
	  ((null? (cdr index)) (set-cdr! index (cdr root-dest)))
	  ((< (caadr root-dest) (caadr index)) (set-cdr! index (cons (cadr root-dest) (cdr index))))
	  (else (add-index-rest! (cdr index)))))
  (if (< (caadr root-dest) (caaar itree))
    (set-car! itree (cons (cadr root-dest) (car itree)))
	(add-index-rest! (car itree))))

(define (add-node! itree root branch)
  (let ((dest (find-elm itree root)))
    (if (null? dest)
	  #f 
	  (begin 
	    (set-cdr! dest (cons (list branch) (cdr dest)))
	    (add-index! itree dest)
		#t))))
(define (find-elm itree root)
  (let ((index (car itree))
        (dest #f))
    (for-each (lambda (elm) (if (= (car elm) root) (set! dest elm) #f))
	          index)
    dest))
	
(define a '(() ((0))) )
(set-car! a (list (caadr a)))