def add_pref_label(value)
      return value if value.providedLabel.empty?
      return value unless value.get_values(RDF::SKOS.prefLabel).empty?
      value.set_value(RDF::SKOS.prefLabel, value.providedLabel.first)
      value
    end
def add_pref_label(value)
      return value if value.providedLabel.empty?
      return value unless value.get_values(RDF::SKOS.prefLabel).empty?
      value.set_value(RDF::SKOS.prefLabel, value.providedLabel.first)
      value
    end
Returns the same value originally given. If a `skos:prefLabel` is not
 present, one is derived from the first `providedLabel` (if any).

 @param [ActiveTriples::Resource] value  the resource to enrich
 @return [ActiveTriples::Resource] the original value, after adding a
   prefLabel