How do you implement a recursive walker of a tree with a lazy range?

[Chris Cain]

On Wednesday, 30 October 2013 at 12:55:41 UTC, bearophile wrote:
> I have used your nice idea to create another partial D solution 
> for this task:
> http://rosettacode.org/wiki/Tree_traversal#D
>
> ... snip ...

Very cool! It's pretty close to being done. I'd have to give some 
real thought to how a level-order traversal might be done lazily 
using D. But that's a good start.