public function subquery( QueryBuilder $queryBuilder ) {
		$this->subqueries[] = $queryBuilder->getSPARQL( false );
		$this->usageValidator->trackDefinedVariables( implode( ' ', $queryBuilder->getSelects() ) );

		// @todo temp hack to add AS definitions to defined variables
		$regexHelper = new RegexHelper();
		$matches = $regexHelper->getMatches( 'AS \{variable}', implode( ' ', $queryBuilder->getSelects() ) );
		$this->usageValidator->trackDefinedVariables( $matches );

		return $this;
	}