I agree. I think the LSP makes it essentially impossible to subclass a concrete superclass, because the subclass must retain all the observable behaviour of th

I agree. I think the LSP makes it essentially impossible to subclass a concrete superclass, because the subclass must retain all the observable behaviour of the superclass. If it does so then what's the point? On the other hand, implementing an interface is fine. Different implementations of the interface can just uphold the interface invariants without needing any relationship to each other.