Add spanning tree proof

I heard that @amintimany has a port of FCSL's spanning tree algorithm to Iris. That would be a good candidate to be added here, wouldn't it? @amintimany, which version of Iris is it working with?

https://github.com/amintimany/iris-spanning-tree

assigned to @amintimany

mentioned in merge request !3 (merged)

closed via commit f4246d3a

closed via merge request !3 (merged)