This is a big refactoring; the parser is now modal, to handle either
complex Node objects that carry around a lot of state with them, or
simpler objects that represent the McCarthy-style IST. I'm still
feeling my way through the subject material. The node construction
is such pure artifice I feel silly keeping it, and may end up revising
it (again).
The nice thing is, everything goes through evaluate. Almost no other
code needs to know anything at all about the shape of the Nodes themselves;
it all makes assumptions based on the return value (or continuation passed)
by evaluate.
Elf M. Sternberg