(define unparse
  (lambda (exp)
    (cond
     ((lit? exp) (lit-datum exp))
     ((varref? exp) (varref-var exp))
     ((function? exp)
      (list 'lambda 
	    (list (function-formal exp)) 
	    (unparse (function-body exp))))
     ((app? exp) (list 
		  (unparse (app-rator exp))
		  (unparse (app-rand exp))))
     (else (error 'unparse  ": Invalid abstract syntax" 
                               exp )))))