(define-record-type lit (fields datum))
(define-record-type varref (fields var))
(define-record-type  app (fields rator rands))

(define parse
  (lambda (datum)
    (cond
     ((number? datum) (make-lit datum))
     ((symbol? datum) (make-varref datum))
     ((pair? datum) (make-app (parse (car datum)) (map parse (cdr datum))))
     (else (error 'parse "Invalid concrete syntax " datum)))))