So nil would be an ubertype that basically just means "empty"? One issue with that is (is 0 nil) would be t... I think it's useful to not always treat 0 as the same as nil.
Not sure about empty tables, though... maybe it would be fine to treat those as nil. We already treat an empty list as nil, after all.
Actually, what you're suggesting is very similar to how languages like JavaScript and Python do it, with 0 null undefined NaN "" etc. being falsy. So in JS, you can do this:
var foo = 0;
if (foo) {} // will not be evaluated
foo = "";
if (foo) {} // will not be evaluated
foo = NaN;
if (foo) {} // will not be evaluated
On the other hand, both those languages support actual Booleans, and they treat an empty array as true.
