[plt-scheme] HtDP 18.1.13 Help Requested

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