Should that test be more strict, and prevent odd and even layers to be set at the same time?
For instance, if we have a 4-layer GSN, and we try to set layers 0 and 3, it's not clear to me what would happen first: computing layer 1 given 0, then 0 and 2 given 1 and 3? or computing layer 2 given 3, then 1 and 3 given 0 and 2? (I'm trying to attach a drawing of this.)

![photo](https://f.cloud.github.com/assets/178249/1151593/63b98624-1efd-11e3-84de-401296b7ea20.JPG)