310
311
312
313 - tangling primitives
314 - better syntax for functions
315 - handle before/after uniformly like def
316
317
318
319 - ack, forgot to handle blocks when tangling

Will the 'lightweight tools' really be all that useable if we encourage
people to layer them one atop another and track precisely what inputs
each can accept? Something to keep an eye on.

In the meanwhile, we have a new (but very unrealistic) example
demonstrating the tangling directives.

There's still a big constraint on ordering: before/after clauses have to
come before functions that need them.
320
321 - before/after can now come anywhere

This pollutes our traces with all 'system software'. Too much trouble to
keep it out; just lump it for now. Who knows, might even be useful. Most
of the time convert* labels are easy to grep out when debugging.
322
323 - function clauses

You can now call 'def' on a function name multiple times. Each time the
instructions you provide are *prepended* to any existing instructions.

One important use for this is when you define a new type and need to
support it in all your generic methods. Now you can keep all those
extensions in one place, near the definition of the type.

To redefine a function, use 'def!'.
324 - more elaboration in Readme

I have different prose in Readme and at the top of the tests. Let's see
how hard they are to manage over time.
325 - example program using channels
326