public static function getChildComponentClass($class, $generator, $componentKey = null)
    {
        $constraints = array(
            'generator' => $generator,
        );
        if ($componentKey) $constraints['componentKey'] = $componentKey;
        $classes = array_values(self::getChildComponentClasses($class, $constraints));
        if (!isset($classes[0])) {
            if (!$componentKey) {
                throw new Kwf_Exception("no component for generator '$generator' not set for '$class'");
            } else {
                throw new Kwf_Exception("childComponentClass '$componentKey' for generator '$generator' not set for '$class'");
            }
        }
        return $classes[0];
    }