#lang racket
; CS 251: programs as data in Racket

(define (is-addition? e)
  (eq? (car e) '+))

; Return #t if the quoted expression e
; contains an addition expression at
; the top level or as a subexpression.
(define (contains-addition? e)
  (cond [(null? e) #f]
        [(cons? e) (or (contains-addition? (car e))
                       (contains-addition? (cdr e)))]
        [(eq? '+ e) #t]
        [#t #f]))


(define (contains-addition? e)
  (cond [(null? e) #f]
        [(and (cons? e) (eq? (car e) '+)) #t]
        [(cons? e) (or (contains-addition? (car e))
                       (contains-addition? (cdr e)))]
        [#t #f]))












; Create a new quoted expression just like e,
; except that all occurences of varx (some
; symbol) are replace with vary (another symbol).
; Think carefully about shadowing...
(define (subst varx vary e) e)