#lang racket

(require racket/trace)

(define arithmetic
  (lambda (n proc)
    (proc n 42)))

(define member?
  (lambda (e ls)
    (if (null? ls)
        #f
        (if (pair? (car ls))
            (if (member? e (car ls))
                #t
                (member? e (cdr ls)))
            (if (eq? e (car ls))
                #t
                (member? e (cdr ls)))))))

(define member2?
  (lambda (e ls)
    (if (null? ls)
        #f
        (if (pair? (car ls))
            (or (member2? e (car ls))
                (member2? e (cdr ls)))
            (or (eq? e (car ls))
                (member2? e (cdr ls)))))))

(define member3?
  (lambda (e ls)
    (cond [(null? ls) #f]
          [(pair? (car ls)) (or (member3? e (car ls))
                                (member3? e (cdr ls)))]
          [else (or (eq? e (car ls))
                (member3? e (cdr ls)))])))


(define flatten
  (lambda (ls)
    (cond [(null? ls) '()]
          [(pair? (car ls)) (append (flatten (car ls)) (flatten (cdr ls)))]
          [else (cons (car ls) (flatten (cdr ls)))])))
 
(define flatten2
  (lambda (ls)
    (if (null? ls)
        '()
        (if (pair? (car ls))
            (flatten2 (append (car ls) (cdr ls)))
            (cons (car ls) (flatten2 (cdr ls)))))))
   
(trace flatten flatten2)