Idiomatic way of writing nested loops?

[Sebastiaan Koppe via Digitalmars-d-learn]

On Monday, 17 July 2017 at 11:07:35 UTC, Anton Fediushin wrote:
> Hello! What is the best way of rewriting this code in idiomatic 
> D manner?

https://dlang.org/phobos/std_algorithm_setops.html#.cartesianProduct