Plus42 algebraic expressions update

[Thomas Okken]

9:25 PM EST / 2:25 UTC -- New builds, with several fixes, none of which actually has anything to do with the equation editor...

• RTN from the keyboard now restores the stack mode if it had been changed with L4STK or LNSTK
• The Export Programs dialog in the Windows version would choke on programs with a lot of alpha labels, and show them as blank lines in the program list; fixed
• Implemented PRMVAR, which prints all the MVAR-declared variables for a given label. This is actually somewhat related to the equation editor, since that kind of functionality should be available there as well, for feature parity with the 17B/27S
• Fixed crash when printing empty lists with PRX and PRV
• Fixed possible null pointers on the stack when using APPEND or EXTEND on lists in low-memory situations

Vincent, that last item seems like it may be related to the bug you reported, but I can't say I'm confident about that. The scenario that I fixed requires getting an Insufficient Memory error from APPEND or EXTEND before weird objects appear on the stack, i.e. the low-memory situation is what causes the problem, it's not the result of it.