write once type?

[sighoya]

On Tuesday, 20 April 2021 at 19:56:33 UTC, Steven Schveighoffer 
wrote:
> Not only that, but I may also want to keep processing the loop 
> and do something different if the value has already been set 
> instead of returning immediately, which necessitates a second 
> loop.

Can I ask why you require to handle it in one loop?
As I see it, you are required to poll over isSet/isNotSet in 
every iteration.