How to use sets in D?

[tastyminerals]

https://forum.dlang.org/post/mailman.1072.1581112984.31109.digitalmars-d-learn@puremagic.com

On Friday, 7 February 2020 at 22:03:00 UTC, H. S. Teoh wrote:
> On Fri, Feb 07, 2020 at 07:37:08PM +0000, mark via 
> Digitalmars-d-learn wrote:
>> [...]
>
> bool[E] works just fine.
>
> The bool does take up 1 byte, though, so if you're *really* 
> want to optimize that away, you could do this:
>
> 	alias Unit = void[0];
> 	enum unit = Unit.init;
>
> 	// Look, ma! A bona fide set!
> 	Unit[E] mySet;
>
> 	mySet[...] = unit;
> 	mySet.remove(...);
> 	... // etc.
>
> Or you can wrap void[0][E] in a nice user-defined type that 
> gives nice set-like syntax.  But IMO, this is all overkill, and 
> adds needless complexity. Just use bool[E] or 
> std.container.rbtree. :-D
>
>
> T

Can you please explain what does `bool[E]` mean? And how does the 
code with aliasing void[0] and then using enum even work?