WIP: 16C firmware hack for more memory

[brouhaha]

I'm working on adding the basic statistical functions (CLR Σ, Σ+, Σ-, mean, sample std. dev.) to the 16C. I recently posted a survey of what numbered reegisters are used for statistical accumulation in various HP calculators. In that post, I said that I was considering using registers 0 to 5, like the 10C, 11C, and 34C. However, after thinking this through a bit more, I'm now considiering registers .0 through .5 (indirect 16 through 21) like in the 19C/29C, 323E, 55, 91, and 95C. My thought is that they're still available without indirect addressing, but leave the low numbered registers available for other uses.

I could possibly add a ΣREG function like the 41C/42S, to allow them to be moved, in which case I think I'd still have them default to starting with .0, and also, like most models but unlike the 41C/42S, have n be the first statistical register rather than the last.

What do those of you that might care about a 16C with statistical functions think?

The Σ+ and Σ- functions would the f-shifts of + and -, though only in FLOAT mode. In integer modes those f-shifts would still be OR and NOT. I haven't decided on key assignment for CLR Σ, mean, sample std. dev., and ΣREG, but it would probably make sense to have those as the f- and g-shifted functions of × and ÷, to keep all the stats functions in the same area of the keyboard.

How important would it be to add L.R., ŷ,r, or weighted mean?

It appears that the stack behavior for Σ+ and Σ- are the same for all HP calculators with two-variable statistics, but there seem to be some differences in stack behvaior for L.R, ŷ,r, etc. between models, even within the Voyager series. I haven't checked the actual calculators, but the HP-11C manual says L.R. stores X into LASTx, and lifts Y and Z into Z and T. The 15C manual says that it lifts X and Y into Z and T. I would have expected these models to behave identically.