const challenge

[Walter Bright]

There are some research papers about unique as a type qualifier, and it 
was toyed around with for a while with D. The trouble with it was:

1) complexity (yet another type qualifier)

2) it still never was quite able to get rid of all the interesting cases 
where a cast(invariant) (i.e. at some point trust the user) was needed

3) it was hard to understand

4) its utility seemed very limited