A nice D coding pattern

[Meta via Digitalmars-d-learn]

That's a neat trick, although if preconditions were able to be 
run at compile time when possible you wouldn't have to resort to 
using enum to force CTFE (you've talked a bit about this before I 
remember). Thinking about something like a good ranged number 
implementation, we can now get almost all the way there. Manually 
having to specify enum is still an undesirable.