public function where($field, $section = 'Reference') {
		// default to reference
		if($section === null) {
			$section = 'Reference';
		}
		// array_values reindexes the array here, otherwise we have gaps from non-matching array keys
		return new FilterableList(array_values(array_filter($this->sections, function(Section $val) use ($section) {
			// only return matching root sections
			return $val->getHeading() == $section;
		})), $field);
	}