scheme stuff

shadows.rkt

#lang racket

(define (atom? item)
  (not (pair? item)))

(define (numbered? aexp)
  (cond [(atom? aexp) (number? aexp)]
        [else 
         (and (numbered? (car aexp))
              (numbered? (car (cdr (cdr aexp)))))]))

(define (value nexp)
  (cond [(atom? nexp) nexp]
        [(eq? (car nexp) '+)
         (+ (value (car (cdr nexp)))
            (value (cdr nexp)))]))