How to declare "abstract" delegates list?

[bauss via Digitalmars-d-learn]

On Saturday, 6 May 2017 at 06:07:01 UTC, bauss wrote:
> On Friday, 5 May 2017 at 14:20:43 UTC, RedCAT wrote:
>> [...]
>
> I would do something like this:
>
> [...]

You could also do use alias this to use the delegate instead of 
the class encapsulating the delegate.