validation by number of matches for list

[rmpt]

Is your feature request related to a problem? Please describe.

I have a list of abstract instances and I want to validate the exactly 1 and only a element is of subtype X.

Describe the solution you'd like

To validate 1 match, I have to do something like:

validate(SomeEntity::myList)
    .validate { it.single { listItem -> listItem is MySpecificSubClass } }

For N matches I would have to add a counter or something.

Describe alternatives you've considered

I have to keep like an external counter or so.

Additional context

No additional data