[racket] Visualization of Exercise 12.4.2 (arrangements)

Hi,

I solved Exercise 12.4.2 (the arrangements problem) after writing clear
purpose statements and writing a few different auxiliary functions. It was
an eye-opener for relying on wishful thinking and trusting that recursive
calls would work as intended.

Exercise 17.6.5 (gift-pick) calls for us to use 'arrangements' again, and
after changing the variable names to make it a general-purpose function, I
realized that I'm still having trouble visualizing a flow-chart. I don't
want to spoil the experience by emailing my solution to people who may
still be working on the problem, but could someone who has a good handle on
the problem please review my code and help me visualize the flow?

Thanks,
Mike
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