Recommended ways to handle final classes

[Dicebot]

It very is likely that you in fact want A to be a struct and _A - 
template mixin.

In other words, you are trying to express a relationship that 
does not really model anything in OOP terms. You proposal will 
work but it will essentially be just hacky implementation of 
mixins with some extra informational overhead.

Maybe we can come with a better proposal if you explain what 
object model are you trying to achieve?