'Unknown' profile metadata
When a piece of metadata in the profile needs to be specified as
unknown, use metaDescribes with predicate="nuk" and value
unknown.
metaDescribes[@predicate='nuk'][.='unknown']
To show the education of the subject of the profile is unknown:
<metaItem about="education">
<metaDescribes predicate="nuk">unknown</metaDescribes>
</metaItem>