QVT14 — QVTr: Rule Overriding semantics

The abstract syntax of QVTr allows a rule to be an override of another rule.

Rule::overrides: Rule [0..1]
The rule that this rule overrides.

The concrete syntax of QVT allows it too:

<relation> ::= ['top'] 'relation' <identifier>
['overrides' <identifier>]
'

'

However, the only semantics I can see for 'overrides' is in clause 7.11.1.4 that says:

"A rule may conditionally override another rule. The overriding rule is executed in place of the overridden rule when the overriding conditions are satisfied. The exact semantics of overriding are subclass specific. "

Questions:

1- What are the overriding conditions? are they implied or specified and if so how?

2- I have not seen any other discussion of overriding in a subclass or Rule so not sure what is meant by "The exact semantics of overriding are subclass specific"?

3- I have not seen any example of using 'overrides' what so ever in the spec, shouldn't there be one?

4 - What is the semantics of overriding? is it related to inheritance in the OO sense ? I think QVTr needs a good "inheritance" model where you can relations can be called polymorphically.