Non-unique EList to support same object multiple times

[riemi-nie]

P21 files support declarations like this:

#572= IFCBSPLINESURFACEWITHKNOTS(3,3,((#573,#574,#575,#576,#573,#574,#575),(#577,#578,#579,#580,#577,#578,#579),(#581,#582,#583,#584,#581,#582,#583),(#585,#586,#587,#588,#585,#586,#587)),.UNSPECIFIED.,.F.,.T.,.F.,(4,4),(1,1,1,1,1,1,1,1,1,1,1),(0.0,15.4213505620632),(-3.0,-2.0,-1.0,0.0,1.0,2.0,3.0,4.0,5.0,6.0,7.0),.UNSPECIFIED.);

Currently it is not possible to store an IfcCartesianPoint twice in list attribute of IfccartesianPointInList.

[bekraft]

Unique lists are the default in xcore. Actually, the "unique" keyword seems not to have any impact.
See:https://www.eclipse.org/forums/index.php/t/1066745/

The solution is an additional ecore annotation to that reference (bug 467855).
See: https://bugs.eclipse.org/bugs/show_bug.cgi?id=467855

Will be added next.

[riemi-nie]

Now I got multiple error inside Eclipse complaining:

A containment or bidirectional reference must be unique if its
upper bound is different from 1

No source code is generated.

[riemi-nie]

The problems are the bidirectional opposite references of xcore.