; CONTROLLER STUFF:

; Control and drive the simultaneous pre-order traversals of tr1 and tr2.
; Return a list of pairs (n1 . n2) where n1 comes from tr1 and n2 comes from tr2.
; Stop when one of the trees are exhausted. 
(define (traverse-controller tr1 tr2)
  (let ((tr1-point (call-with-current-continuation (lambda (here) (traverse-start tr1 here))))     
        (tr2-point (call-with-current-continuation (lambda (here) (traverse-start tr2 here)))))    
    (letrec ((tree-2-stepper                                                                       
              (lambda (tr1-point tr2-point)  ; tr1-point and tr2-point are each a pair:            
                                             ; (tree-node . traversal-cont)                        
                (if (end-traversal? tr1-point tr2-point) 
                    '()
                    (let ((n1 (car tr1-point))
                          (c1 (cdr tr1-point))
                          (n2 (car tr2-point))
                          (c2 (cdr tr2-point)))
                       (cons (cons n1 n2)                                                          
                          (let ((tr1-nxt-point (call-with-current-continuation                     
                                                  (lambda (here) (c1 here))))                      
                                (tr2-nxt-point (call-with-current-continuation                     
                                                  (lambda (here) (c2 here)))))
                            (tree-2-stepper tr1-nxt-point tr2-nxt-point ))))))))                   
       (tree-2-stepper tr1-point tr2-point ))))

(define (end-traversal? x y)                                                                       
  (or (and (pair? x) (not (car x)))                                                                
      (and (pair? y) (not (car y))) ))                                                             

; (traverse-controller tr1 tr2) =>
; ((2 . 1) (4 . 9) (3 . 0) (7 . 7) (0 . 8) (9 . 2) (1 . 7) (5 . 6))