public static function defaultStack(string $stackName = null)
    {
        if ($stackName) {
            // If the default stack has already been written to, we
            // will make sure to move the existing default stack
            // to the new stack name, as to not lose any data.
            if (static::hasStack(static::$defaultStack)) {
                static::$stacks[$stackName] = static::stack(static::$defaultStack);
                unset(static::$stacks[static::$defaultStack]);
            }

            static::$defaultStack = $stackName;
        }

        return static::$defaultStack;
    }