Hello,I have been studying the implementation of AnyPublisher in the Combine framework and noticed that the implementation uses an abstract base class PublisherBoxBase and a concrete subclass PublisherBox. I am curious about the design decision behind this approach.

My question is:

1.	What is the advantage of having PublisherBoxBase as an abstract base class instead of directly implementing everything in PublisherBox?
2.	Is there a specific design pattern or principle that this approach is adhering to?

I appreciate any insights or explanations you can provide. Thank you!
related code: