How do I check for a relation in a XPATH constraint?

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I have set up an optional relation between entity A and entitiy B. I want only to retrieve the list of A's for which there is no relation with B. Or retrieve the list of A's for which there is no relation with B and a relation with one particular occurence of B.
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1 answers
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Sorry, I have found it already: [not(RelationnameA_B/Field)]

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