Assume:
Class X implement I1
Interface I1 extends I0
I0 defines the property foo

Write a rule:
I1( foo == .. )
and insert an instance of class X

The condition analyzer will look in classes first, rather than interfaces, assuming that "foo" is provided by X.
The next time an instance of some class Y implementing I1 is inserted,
a ClassCastException will be thrown since X is expected