What would the XML look like in case of a 'null association'? I don't know of a way in which the ITS supports this.
<class> <association nullFlavor='NI' xsi:nil='true'/> ... </class>
There is no concept of a null association, but there is the concept of an association to a null object. So the XML would look like this:
<class> <association nullFlavor='NI'/> </class>
So, there is no ...: if the object is null there can be no elements (though attributes such as classCode and moodCode could be appropriate) And xsi:nil would not be appropriate, though this is a matter of interpretation