hc-append unbound identifier in module in
===

Hello I am following [the quick introduction to racket][1]. In chapter 5 they propose that this code (which worked fine):

    (define (four p)
      (define two-p (hc-append p p))
      (vc-append two-p two-p))

would tipically be written by using let, so I wrote my code:

    (define (four p)
      (let ([two-p (hc-apppend p p)])
      (vc-append two-p two-p)))

but it doesn't work. 
The error message is:
       
    expand: unbound identifier in module in: hc-apppend


  [1]: http://docs.racket-lang.org/quick/