[chapter 6] Notes for chapter 6

I'm pretty OK with recursion, so not too much to record here.

What's in the spec comes from a few foot notes that are on 122 and 123,
ala:

> 3. A short-circuiting andify function can also be implemented via
> Underscore’s every function. Can you see how?

And..

> 4. The orify function can also be implemented via Underscore’s some
> function.