preparing for const, final, and invariant

[Walter Bright]

Brad Roberts wrote:
> For any non-trivial application, const (and it's stronger brother 
> invariant) is a must to allow me and the people I work with to both 
> specify the behavior of routines in their signature, and to actually 
> provide enforcement.

The combination of const, invariant, final, and scope will allow one to 
be much more expressive on the intended usage of a variable than C++ 
does, along with enforcement of it.

One downside is there's no dipping one's toe into const-correctness. 
It's got to be done whole hog.