Free42 with big stack, test version

[Thomas Okken]

Yes, running programs in big stack mode, when they were written for the four-level stack, is not going to work well. And of course "don't use big stack mode at all" is a straightforward and effective way of avoiding that scenario.

For people who want to use both modes, I'm still thinking about how to support that best without running programs in the wrong mode by mistake. One improvement I'm thinking of would be this: make the 4STK and NSTK functions non-programmable, and make it so that when the return stack is cleared, any outstanding L4STK and LNSTK operations are undone. That way, programs can use whichever mode they want internally, but the calculator will always revert to the user's preferred mode afterward.

Other solutions are also possible, of course, but that one seems reasonably simple and less likely to cause confusion than some others I could think of...