[plt-scheme] HtDP 18.1.13 Help Requested

Use the design recipe Dave! -- Matthias


On Jun 19, 2008, at 11:49 PM, dave yrueta wrote:

> 18.1.13
>
> Before hitting this problem, I was able to successfully apply the
> ‘backtracking’ technique to solve earlier HtDP exercises such as
> 14.2.2, 15.3.4, 16.3.4. and 18.1.12. However, the ‘Pragmatics’
> discussion leading up to exercise 18.1.13 left me utterly dazed and
> confused.  As a result, I have very little sense of how to ‘backtrack’
> using local expressions.
>
> My best guess is just a start….
>
> (define(to-blue-eyed-ancestor a-ftn)
> 	(cond
> 	[(symbol=? ‘blue (child-ec  a-ftn)) empty]
> 	[(and(empty? (child-m a-ftn))(empty? (child-f a-ftn))) false]
> 	[else…]))
>
> Again, I have no idea how a local expression would fit into a
> solution.  Any and all help is appreciated.
>
> Thanks,
> Dave Yrueta
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