Trait for "can be instantiated"?

[Ben Jones]

On Wednesday, 11 May 2022 at 12:29:05 UTC, Basile B. wrote:
> How about being more explicit in the UDA ?
> The idea would be to associate the enum value to a type or not:

I think that could work but would require some major changes to 
my existing code.  Also, I think I'd prefer:

```
@Token{

  enum lparen = '(';
  enum rparen = ')';
  enum if_token;
}
```

to what you suggested as a user of the library.