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\* Added support for property skosext:candidateLabel. Label properties
mapped to this will be converted to skos:prefLabel if one doesn't exist,
but skos:altLabel if a prefLabel already exists (for the same concept,
in the same language). This makes it possible to prefer some kind of
labels, or labels from a particular source, when there may be many
sources of labels. This is useful for combined ontologies such as TERO
and LIITO, where the "same" concept may have different prefLabels from
the different source vocabularies.