code: public function visit(Criterion $criterion, CriterionVisitor $subVisitor = null)
      {
          return 'location_visible_b:' . ($criterion->value[0] === Criterion\Visibility::VISIBLE ? 'true' : 'false');
      }
docs: Map field value to a proper Solr representation.
  
  @param \eZ\Publish\API\Repository\Values\Content\Query\Criterion $criterion
  @param \EzSystems\EzPlatformSolrSearchEngine\Query\CriterionVisitor $subVisitor
  
  @return string