We defined the terms Backbone Tree and Demand Graph, however I don't remember how those terms are defined and I don't know how to construct them.
Using those structures, one can construct the Network Matrix (I guess kind of a incidence matrix?), which is totally unimodular. Actually, we defined Backbone Tree and Demand Graph to prove total unimodularity of matrices.
Unfortunately, I am not able to find those terms anywhere but in my lecture notes, so maybe they're commonly known under an other name? Fortunately, I remember the concept was quite simple, however I forgot it :(