[racket] A puzzle

From: Mark Engelberg (mark.engelberg at gmail.com)
Date: Wed Jan 19 19:26:23 EST 2011

Hmm, I don't think the greedy approach will work.
The first line of the second thread,  (set! a (list c)), will execute
without error immediately, but it really needs to happen between the
evaluation of a and (first a) in the first line of the first thread.

Probably needs to be done by hand.

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