public static function get_mandatory_properties() {
        $properties = self::define_properties();
        $mandatoryproperties = array_filter($properties, function($property) {
            return empty($property['optional']);
        });
        return array_keys($mandatoryproperties);
    }
public static function get_mandatory_properties() {
        $properties = self::define_properties();
        $mandatoryproperties = array_filter($properties, function($property) {
            return empty($property['optional']);
        });
        return array_keys($mandatoryproperties);
    }
public static function get_mandatory_properties ( ) { $ properties = self :: define_properties ( ) ; $ mandatoryproperties = array_filter ( $ properties , function ( $ property ) { return empty ( $ property [ 'optional' ] ) ; } ) ; return array_keys ( $ mandatoryproperties ) ; }
The list of mandatory properties required on the question object to
export.

@return string[] List of properties.
The list of mandatory properties required on the question object to export .