HUTN — 6.5 : Missing containment information when referencing class instances

When referencing an instance (ClassInstanceRef), the user needs to look at the metamodel in order
to know if it is a contained reference or a non contained reference (BNF rules 22 and 23).
Suggestion 1 for resolution: Use a "forward" prefix keyword when referencing contained instances that
are fully represented in another place (that are not inlined).
Suggestion 2 for resolution : Use a "ref" keyword when referencing non contained instances and use no
keyword to reference contained instances not inlined.
Note: The resolution of this issue may depend on the resolution of "need to improve readability when
referencing instances"