Thanks for these examples – they are good options to have.
I think my question was not quite asking what I wanted to ask precisely.
Actually, I want to _add_ the behaviour in the subclass. The baseclass has no reason to have “Behaviour on opacity”. A later subclass may need Behaviour on width (as an example).
Could I add this behaviour in the subclass?
(That’s what the example not-correct code that I gave was trying to illustrate)