50g System RPL list processing

[Claudio L.]

'?' seems to be much more versatile than I thought. I learnt something new today.
However, if we use it for ΣLIST, then we are changing its definition from undefined to something closer to "nil".
Not that '?' is that useful anyway, but before changing its meaning I need to learn the full extent of the consequences, and make sure nothing breaks in he CAS down the road because of us trying to fix lists. Since I'm not too familiar with it, I'm having second thoughts on using it.

The concept of 'one' does have the trouble of having to define the additions of 'one', and yes, it would only be useful in the context of ΠLIST, as no other command would need it.
But if we don't do it, then ΠLIST returns an error for empty lists, but ΣLIST does not?
Doesn't feel consistent, either both return an error, or both return some abstract value.