This is ready for a full review now. I don't think any new test cases are required.
I haven't looked very deeply into the potential use of an unpure-pure container. It looks like unpure-pure containers are used for vertical placement, and this is horizontal. If that answer is not satisfactory, let me know and I'll spend more time learning about it, but I'd rather work on other things. https://codereview.appspot.com/547340043/
