[plt-scheme] HtDP: Exercises 7.5.*

Hi,

Was sub-chapter 7.5 a point of contention?

I like contracts, but it doesn't feel like it fits.

Evidence:
* Less than 3 pages
* We are encouraged to raise errors, but we haven't got a unit-testing
tool to actually check if they were raised.

I argued that the goal is to show that we grok how to provide checked
versions of our functions to clients, so function documentation and
unit tests aren't required here.

Best wishes,

Grant