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The ideas in this chapter were first presented in Liberating Data
Refinement [22].
The consequences of allowing extra operations in classes for behavioural
subtyping are also studied by Liskov and Wing [148]. Their
``constraint rule" is closely related to our alphabet extension rule.
This issue is discussed in an Object-Z like context by Helke and Santen
[113].
E.A.Boiten
2002-11-22