automatic subtyping/intensions

[Tril]

On 9 Feb 1999, Laurent Martelli wrote:
> I agree that semantics is what matters. Rather than relying on having
> methods or attributes with the same name or signature à la Smalltalk.
> 
> But I think that it is wrong to talk about subtyping between a
> specification and an implementation. I think that subtyping is a
> behavioral notion (and so do Liskov and Wing in [1]).
> 
> 
> 1 - B. Liskov and J. Wing: "A Behavioral Notion of Subtyping", ACM
> Transactions on Programming Languages and Systems, Number 16(6),
> pp. 1811-1841, ACM Press, 1994.

Could you summarize the content of the article?  What is meant by a
"behavioral notion?"

David Manifold <dem@tunes.org>
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