(define entering
  (lambda (test input cond-clause-number)
    (begin
      (if test (writeln "   Entering cond-clause-"
                 cond-clause-number " with tree = " input))
      test)))

(define leaving
  (lambda (result cond-clause-number)
    (begin
      (writeln "Leaving cond-clause-" 
        cond-clause-number " with result = " result)
      result)))

(DEFINE tdepth ; TYPE: (-> ((tree atomic-item)) number)
  (LAMBDA (tree)
    (COND
     [(entering (null? tree) tree 1)
      (leaving 0 1)]
     [(entering (pair? (car tree)) tree 2)
      (leaving
       (max (add1 (tdepth (car tree)))
	    (tdepth (cdr tree)))
       2)]
     [(entering #t tree 3)
      (leaving (max 1 (tdepth (cdr tree)))
	       3)]
     )))