[asdftemp]

https://en.wikipedia.org/wiki/Kirchhoff%27s_theorem ?

[kmill]

Implementation: https://gist.github.com/kmill/f4f47913d036fce687bc

Though it counts reflections as distinct.

[ginko]

It surprised me that he even mentioned the concept of a spanning tree, yet didn't use it for more than a coarse upper bound.