OCL21 — no explanations about how to manipulate optional and multivalued attributes

There are no explanations about how to manipulate optional and multivalued attributes. On the other hand optional and multivalued associations are discussed in detail. For example, while I can use context Person inv: self.job->notEmpty() implies ... to test "whether there is an object or not when navigating the association", I do not know how do a similar test for optional attributes. Is it context Person inv: self.maidenName <> `' implies ... or context Person inv: self.maidenName -> notEmpty() implies ... ?

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