lib/Query/Location/CriterionVisitor/Location/IsMainLocation.php
public function visit(Criterion $criterion, CriterionVisitor $subVisitor = null)
    {
        return 'is_main_location_b:' . ($criterion->value[0] === Criterion\Location\IsMainLocation::MAIN ? 'true' : 'false');
    }
public function visit(Criterion $criterion, CriterionVisitor $subVisitor = null)
    {
        return 'is_main_location_b:' . ($criterion->value[0] === Criterion\Location\IsMainLocation::MAIN ? 'true' : 'false');
    }
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