Feature request: please make it possible for ΣLIST to behave like in NewRPL

[jthole]

In DB48X, and in the HP RPL calculators, list sums of nested lists behave in the following way:

{{1 2}{3 4}{5 6}}
ΣLIST
{1 2 3 4 5 6}

However, in NewRPL, nested lists are handled in the following way:

{{1 2}{3 4}{5 6}}
ΣLIST
{9 12}

In general, in NewRPL nested lists are "summed down" one level. This is very useful when you want to add the first, second, third, etc. element of each sublist. In the example above, 1+3+5=9 and 2+4+6=12.

Please consider implementing both behaviors (HP and NewRPL), possibly switchable via a flag setting.