The semantics of the 'unique' constraint in ADL / AOM have not been fully defined. The child objects of a container attribute in openEHR can never be literally identical, since at least the name differs. Therefore, 'unique' must refer to a logical uniqueness of values. For complex objects this is probably mostly meaningless, since sibling complex objects in a container will almost always differ in some detail. For leaf objects, or near-leaf objects it could have some meaning. It could potentially be formalised by the introduction of an '=' operation on data objects.