(define (implicit-begin exp)
  (cond ((null? exp) 'undefined)
        ((null? (cdr exp)) (car exp))
        (else (cons 'begin exp))))

(define (cond-expand% args)
  (cond ((null? args)
         (error "Invalid-syntax"))
        ((eq? (caar args) 'else)
         (implicit-begin (cdar args)))
        (else
         (list* 'if (caar args)
                (implicit-begin (cdar args))
                (if (null? (cdr args))
                    '()
                    (list (cond-expand% (cdr args))))))))

(define (quote-expand args)
  (define (expand-list lst acc)
    (cond ((pair? lst)
           (expand-list (cdr lst) (cons (quote-expand (list (car lst))) acc)))
          ((null? lst) (cons 'list (reverse acc)))
          (else
           (cons 'list* (reverse (cons (quote-expand (list lst)) acc))))))
  (if (pair? (car args))
      (expand-list (car args) '())
      (cons 'quote args)))

(define (macroexpand exp)
  (cond ((not (pair? exp)) exp)
        ;; syntax
        ((eq? (car exp) 'quote)
         (quote-expand (cdr exp)))
        ((eq? (car exp) 'if)
         (list* 'if (macroexpand (cadr exp)) (macroexpand (caddr exp))
               (if (null? (cadddr exp))
                   '()
                   (list (macroexpand (cadddr exp))))))
        ((eq? (car exp) 'lambda)
         (list* 'lambda (cadr exp) (map macroexpand (cddr exp))))
        ((eq? (car exp) 'set!)
         (list 'set! (cadr exp) (macroexpand (caddr exp))))
        ;; macro
        ((eq? (car exp) 'cond)
         (cond-expand% (cdr exp)))
        ;; procedure
        (else (map macroexpand exp))))