public static function addBuilder($appId, $builderFunction)
{
if (array_key_exists($appId, static::$builders)) {
throw new Exception\FacadeException("Builder with {$appId} already exists!");
}
if (!is_callable($builderFunction)) {
throw new Exception\FacadeException("Builder function for {$appId} is not callable!");
}
static::$builders[$appId] = $builderFunction;
return true;
}