OCL2 — OCL 2: Can collections contain void/undefined objects

Is it possible for collections to contain void (= undefined objects)?

Conceptually, this would appear to be required so that you
can specify whether an item in a collection can be null in
an actual implementation. The use of void and undefined() is
advised in exactly the same situation in a non-collection
context.

However, this quote from Section 2.4.11: "In general, an expression
where one of the parts is undefined will itself be undefined", along
with the rest of the section, shows that you can't use void as
a parameter to the collection calls, and in the context of OCL as a
language, this makes sense.

So I thank that this constraint:

context Collection
inv: self->forAll(not OclIsUndefined())

is required, and this should be stated to clear up uncertainties.

This leaves the problem of how to say whether an "item in a collection
can be null in an actual implementation" is inresolvable in OCL 2.

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