One more question on dependents: how will DependentsArray have nil elements ?

Not via the #removeDependent: message that would call #reject: and answer a 
copy without the nils.

Apart if you add explicitly nil (or implicitly via a variable) in dependents, 
i do not see yet how there would be a nil.

In this case, there is just some useless code, and the bug i did figure will 
never show up.
But i may miss something...