unimplemented abstract function compiles.

[Eric]

I thought it would work the same way as an interface (which must 
be implemented by the direct sub class, otherwise compile error).
But apparently it's possible to implement an abstract function 
anywhere in the class hierarchy. That makes it, in this case, 
impossible to check during compile time.

I ran into this while loading objects from a file using 
Object.factory and forgot to implement an abstract function in 
one class.