iota steps, nothrow, naturals and more

[bearophile]

> A hypothetical solution is to carry around the interval of 
> immutable values between lines and use the contracts to shape 
> such intervals, as asked here:
> https://d.puremagic.com/issues/show_bug.cgi?id=10594

Please vote the issue, now you have 20 votes :-)
Pre-state (old), the shaping of in/immutable value ranges with 
contracts, and few more things make D contract programming more 
useful.

Bye,
bearophile