Mon Oct 24 19:15:38 EDT 2011

Zipper for Expr?

Maybe that's not even necessary.  There is a direct correspondence
between the functions from the type class rep and the constructors so
we don't really need to turn trees inside out, or do we?  Yes we do.
At least for "let".

Anyways, a more important change is to abstract the monad in TML, so
that we can implement TML in terms of Term only (just an environemnt)
but also Expr.  Or maybe just switch completely to Expr?

( It's crazy how this is so little code, but so much work to write it!
Because of the seemingly unlimited compositional freedom in haskell of
course things that look almost the same are not quite. )