[plt-scheme] Novice needs help writing function

The design recipes are a (if not the) core part of HtDP.  HtDP is the
only book I know that takes designing programs from a dark and
mysterious art to a systematic process anyone can apply.  This is a
really really powerful idea.  So, if you haven't worked through the
design recipe, do so.  If you have, then you might say what
difficulties you are having applying the recipe.

N.