> the subtype passes all black-box tests designed for the supertype The problem is that it's possible to write black box tests which fail when any extensions a

> the subtype passes all black-box tests designed for the supertype The problem is that it's possible to write black box tests which fail when any extensions are detected in the object, so you can't reduce it to a glibly stated rule like this. There have to be some blackbox tests that are not allowed.