Hello again... sorry about delay, yestarday was busier than I thought.
Here's what I think the construct should look like (I've used VS2kx's UML Diagram)...

The illustration below, I think is getting closer to what flamandier was expecting, and Greet expressed (thank you by way, I thought I was on quite firm ground) personally I rather like the non-standard notation on the asCollectionAssociation to Y; it gives a visual clue to something being abstracted away... expressing that X does reference Y.

Lots of folks that I work with, expect to have to read the code and model to understand whats happening... little things like implicitly occluding these types of constructs lead to that thinking.
If youre going to reverse engineering from source to model, then we ought to be able to choose what we show in a diagram... else we'll be inspecting text for ever more.
Appologies for my appalling spelling...
Kevin [smiley=2vrolijk_08.gif]
ps. there are 3 fields in X, all defined as List<Y> in the second diagram