hypothetical: more return stack levels on 11C, 15C, 16C

[brouhaha]

For now, I've decided not to change the stack levels. I can revisit that in the future if a good justification comes along.

I need to think more about Thibault's request for displaysegment control. The 41C has a "message flag" that indicates that something special is being displayed, so normally display processing is disabled. The Voyager calculators don't have an equivalent. I can display special stuff during the execution of a running program, and _maybe_ do a special program stop that doesn't refresh the display, until the next keypress. I'm not sure a special display during PSE will work.

Obviously if I have time to study the HP code in enough detail, I could potentially make it do anything the hardware is capable of, but I prefer not to go down a path or really complicated changes.

If all goes well with my rewrite, I hope to publish a demo version of Nonpareil II with a test version of the 16C hacks (expanded memory, trig, inv. trig, logs, exponentials, maybe statistics) in the not-too-distant future.