How to create a Yin Yang in Pixelmator

I have been trying to create a Yin Yang effect in Pixelmator and, whilst I have been relatively successful using a number of well-placed semi-circles, this method fails when I try to add a stroke to my shape, or change its colour.

So this is what I have done so far - please bear with me as I'm fairly inexperienced with graphic design software in general!

4 semi-circles seem to do a good job of creating the effect. However, if you try to add a stroke that will go along the centre curve, the obvious happens:

So I have tried creating a third semi-circle to blank out the lower half of the shape. Ideally I want to find a way of telling Pixelmator to delete this section of the larger semi-circle:

I have merged the layers, although I'm not entirely sure what I should do next. If I try and give this layer stroke, I get this effect:

And if I try and give it a fill, this happens:

Essentially, I wonder if I'm actually going about this completely wrong. I would be extremely grateful if anyone could offer any advice on the best way to achieve this.