*** tree-neg (omitted due to lack of time)
--------------
(tree-neg '()) ==> ()
(tree-neg '(1 2 ())) ==> (-1 -2 ())
(tree-neg '(((5 ()) (7)) 9))
        ==> (((-5 ()) (-7)) -9)

tree-neg: (-> ((tree atomic-item))
               (tree atomic-item))
--------------
 	have them work this in groups

	the idea of this is to capture the pattern
	base case: '()
	flat case: like copy (for lists)
	tree case (car of tr is a pair):
		want to get (tree-neg '((1 2) (3 4))) = '((-1 -2) (-3 -4))
			from (tree-neg (car '((1 2) (3 4))))
				= (tree-neg '(1 2))   = '(-1 -2)
			and (tree-neg (cdr '((1 2) (3 4))))
				= (tree-neg '((3 4))   = '((-3 -4))
		use cons

(define tree-neg
  ; TYPE:  (-> ((tree atomic-item))
  ;            (tree atomic-item))
  (lambda (tr)
    (cond
      ((null? tr) '())
      ((number? (car tr))
       (cons (car tr)
	     (tree-neg (cdr tr))))
      (else (cons
	      (tree-neg (car tr))
	      (tree-neg (cdr tr)))))))