'None' profile metadata
Where we want to specify the absence of a piece of metadata in the profile, use metaDescribes with predicate="nuk" and value none.
metaDescribes[@predicate='nuk'][.='none']
To show the subject of the profile had no spouse:
<metaItem about="spouses">
<metaDescribes predicate="nuk">none</metaDescribes>
</metaItem>