I'm trying to have proto allow me to do things like:
(a,b,c) = expr;
where a, b, c are integers of different width and they should be
assigned the appropriate slice of expr.
if in ...
(a,b,c) = expr;
... a,b,c are of unproxied builtin type, then it might not be possible
in C++, as you can not overload operators (in this case
'operator,(lhs,rhs)') upon builtin types only.