Pie rule

[andyljones]

I've neglected the pie rule so far because it's the one fly in Hex's otherwise admirably clear ointment. But if I wanna use the (#13) checkpoints for benchmarking, it needs to be added in.

[andyljones]

This isn't gonna happen on this iteration of the project; I've run a fair chunk of experiments now without the pie rule, so I'm reluctant to add it and invalidate all those. Might resurrect this in a future iteration.