That’s a good question, I don’t know the history well enough to comment on that. Much more important than CLU itself — groundbreaking as it was — was Liskov’s

That’s a good question, I don’t know the history well enough to comment on that. Much more important than CLU itself — groundbreaking as it was — was Liskov’s formalization of substitutability as a rigorous principle one can use to reason about the quality or correctness of an arbitrary abstraction (or model thereof).