If type vars get into the espression you have to keep them in sync with
the unification or you can lose information.
Some combinators can put symbols on the expression, you have to convert
those to type checkers or, as a hack, just look them up and run them.
This lets definitions work(-ish), ...
So they can notice if they're given a stack that doesn't match what
they're expecting.
This seems to work, but I realized that type variables in the pending
expression need to be update()'d too. hmm...
I'm backfilling tests to cover the functionality that I developed
incrementally in the Jupyter notebook and nail it down with concrete
examples. No doubt I'll uncover some bugs.
Simon Forman