Can someone explain to me the design choices for this function definition?

[e-y-e via Digitalmars-d-learn]

On Thursday, 22 September 2016 at 21:41:38 UTC, Lodovico Giaretta 
wrote:
>> 2. Why is openRight not a Flag type?
>
> It may be that until is quite old, predating the extensive 
> usage of Flag, and maybe also the extensive use of compile-time 
> flags.

Ok, I suspected this might be the case. I might work on a pull 
request for the enum -> Flag item, as I can't think of any reason 
not to. Off the top of my head I don't think it would break any 
existing uses.

Thanks,