I don’t think you’re misunderstanding anything, but to clarify, I’m talking about a more general notion of subtyping (e.g. structural, “duck typing” etc), not n

I don’t think you’re misunderstanding anything, but to clarify, I’m talking about a more general notion of subtyping (e.g. structural, “duck typing” etc), not necessarily inheritance. Instances of A and B can (in Smalltalk parlance) respond to the same set of messages, and therefore be substitutable for each other, without having any inheritance relationship. In that case there’s no coupling of implementation, only the flexibility of being able to swap out an A instance for a B instance without its collaborators being any the wiser.