* Recursion
What other examples do we want to use here?  trees?  have we
introduced pattern matching yet?  there's a bit of a co-dependency
between getting deeply into recursion and getting deeply into pattern
matching.  Maybe recursive types, "recursion isn't just for
functions."  We can do recursive versions of the power function and
such.  More list examples, so we can see the pattern?  Does the IO
monad make these examples more complex than they need to be?
Recursion: see recursion. i = i + 1 is kinda like recursion, it's
self-referential.  A sentence is made up of clauses, which is made up
of words and clauses?  Service your Service Engine Soon light.
Catch-22 (non-terminating), mutual recursion