relax cut set condition

[brendan8229]

Our cut set condition is too strong. The cut set is currently defined to be where nodes are prefix equivalent and suffix equivalent. It is sufficient if the end nodes are suffix equivalent and the start nodes are prefix equivalent.

We'll have to be very careful to correct this, but please start on that.

An example where this is the case is a graph in the photo. a and b are not an eq class, but are suffix equivalent, just like c and d are prefix equivalent.