Create a CALayer mask by combining multiple paths

Question

I'm trying to create a mask for a CALayer containing an image. This mask shall be able to use the union of multiple paths or the inverse of the union. In terms of a venn diagram that would be a disjunction or a joint denial (look here for graphics of the venn diagram that I can't post: https://en.wikipedia.org/wiki/Logical_connective#Common_logical_connectives). I also started a thread in a dedicated iOS-forum with lots of graphics and even more example code: http://www.raywenderlich.com/forums/viewtopic.php?f=2&t=7155

I do succeed to create the first case: a union of multiple paths (disjunction).

You can copy the code into a new single screen app to try it out (don't forget to include the Quartz framework):

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    // create a yellow background
    UIView *bg = [[UIView alloc] initWithFrame:self.view.bounds];
    bg.backgroundColor = [UIColor yellowColor];
    [self.view addSubview:bg];

    // create the layer on top of the yellow background that will be masked
    CALayer *imageLayer = [CALayer layer];
    imageLayer.frame = self.view.layer.bounds;
    imageLayer.backgroundColor = [[UIColor blueColor] CGColor];

    // create the mask that will be applied to the layer on top of the
    // yellow background
    CAShapeLayer *maskLayer = [CAShapeLayer layer];
    maskLayer.fillRule = kCAFillRuleEvenOdd;
    maskLayer.frame = self.view.frame;
    CAShapeLayer *maskSubLayer1 = [CAShapeLayer layer];
    maskSubLayer1.fillRule = kCAFillRuleEvenOdd;
    maskSubLayer1.frame = self.view.frame;
    CAShapeLayer *maskSubLayer2 = [CAShapeLayer layer];
    maskSubLayer2.fillRule = kCAFillRuleEvenOdd;
    maskSubLayer2.frame = self.view.frame;

    // add the paths to sublayers of the mask
    maskSubLayer1.path = CGPathCreateWithRect((CGRect){{100, 140}, {100, 150}}, nil);
    maskSubLayer2.path = CGPathCreateWithEllipseInRect((CGRect){{80, 40}, {190, 190}}, nil);
    // add the sublayers to the mask
    // (instead of using CGPathAddPath())
    [maskLayer addSublayer:maskSubLayer1];
    [maskLayer addSublayer:maskSubLayer2];
    // apply the mask to the layer on top of the yellow background
    imageLayer.mask = maskLayer;
    [self.view.layer addSublayer:imageLayer];

}

However, the inverse seems untamable, for instance with a masked mask:

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    // create a yellow background
    UIView *bg = [[UIView alloc] initWithFrame:self.view.bounds];
    bg.backgroundColor = [UIColor yellowColor];
    [self.view addSubview:bg];

    // create the layer on top of the yellow background that will be masked
    CALayer *imageLayer = [CALayer layer];
    imageLayer.frame = self.view.layer.bounds;
    imageLayer.backgroundColor = [[UIColor blueColor] CGColor];

    // create the mask that will be applied to the layer on top of the
    // yellow background
    CAShapeLayer *maskLayer = [CAShapeLayer layer];
    maskLayer.fillRule = kCAFillRuleEvenOdd;
    maskLayer.frame = self.view.frame;
    CAShapeLayer *maskSubLayer1 = [CAShapeLayer layer];
    maskSubLayer1.fillRule = kCAFillRuleEvenOdd;
    maskSubLayer1.frame = self.view.frame;
    CAShapeLayer *maskSubLayer2 = [CAShapeLayer layer];
    maskSubLayer2.fillRule = kCAFillRuleEvenOdd;
    maskSubLayer2.frame = self.view.frame;

    // add the paths to sublayers of the mask
    maskSubLayer1.path = CGPathCreateWithRect((CGRect){{100, 140}, {100, 150}}, nil);
    [maskLayer addSublayer:maskSubLayer1];
    maskSubLayer2.path = CGPathCreateWithEllipseInRect((CGRect){{80, 40}, {190, 190}}, nil);
    [maskLayer addSublayer:maskSubLayer2];
    // use the maskLayer to mask the maskBackgroundLayer
    CALayer *maskBackgroundLayer = [CAShapeLayer layer];
    maskBackgroundLayer.frame = self.view.layer.bounds;
    maskBackgroundLayer.backgroundColor = [[UIColor greenColor] CGColor];
    maskBackgroundLayer.mask = maskLayer;

    // apply the maskBackgroundLayer to the layer on top of the yellow background
    imageLayer.mask = maskBackgroundLayer;
    [self.view.layer addSublayer:imageLayer];
    // just for testing: instead of the 2 lines above look at the maskBackgroundLayer
//    [self.view.layer addSublayer:maskBackgroundLayer];

}

I'm looking for ideas how to achieve this effect.

Similar questions are: iOS - Clipping union of two paths using UIBezierPath appendPath (not applicable in my case since the intersection can be irregular) Combining Intersecting CGPaths on iOS (unresolved)