(define first-of-three
  (lambda (a b c)
    (cond
      ((or (null? a) (null? b) (null? c)) #f)
      (else (list (car a) (car b) (car c))))))

(define prependl
  (lambda (l r)
    (cond
      ((null? l) r)
      (else (cons (car l) (prependl (cdr l) r))))))

(define revl
  (lambda (l)
    (cond
      ((null? l) l)
      (else (prependl (revl (cdr l)) (list (car l)))))))

(define palindrome
  (lambda (l)
    (prependl l (revl l))))


(define union
  (lambda (a b)
    ()))

(define intersection
  (lambda (a b)
    ()))

(define difference
  (lambda (a b)
    ()))

(define duplicate
  (lambda (l)
    (cond
      ((null? l) l)
      (else (prependl (list (car l) (car l)) (duplicate (cdr l)))))))

(define suffixes
  (lambda (l)
    (cond
      ((null? l) l)
      (else (prependl (list l) (suffixes (cdr l)))))))