(define desc  ; TYPE: (-> (s-expression) (tree symbol))
  (lambda (s)
    ; ENSURES: (eval result) = s
    (cond
     ((atom? s)
      (if (self-describing? s) s (list 'quote s)))
     (else (list 'cons (desc (car s))
		       (desc (cdr s)))))))

(define self-describing?  ; TYPE: (-> (atom) boolean)
  (lambda (a)
    (and (not (null? a)) ; () is sometimes a boolean
	 (or (number? a) (boolean? a) (string? a)))))

(define atom?  ; TYPE: (-> (datum) boolean)
  (lambda (x)
    (not (pair? x))))