OCL25 — How does Set distinguish duplicate values?

In 11.6.2: "It [Set] contains elements without duplicates."

What is a duplicate?

In 10.2.2.13 SetTypeValue
"All elements belonging to a set value have unique values.
self.element->isUnique(e : Element | e.value)"

>From 11.9.1.3 isUnique, the basis of comparison is <>:
forAll (x, y | (x.iter <> y.iter) implies (x.value <> y.value))

But what is the Element::value to which "<>" is applied. It is far from clear that the semantics of the Element in Section 10 which is completely unrelated to MOF::Element leads to the obvious answer.

Suggest adding the obvious WFR.

context Set
inv: forAll(x | self->count = 1)

(count is already defined using "=")

[And chnage 11.9.1.3 isUnique to use this much more readable formulation.]