I sketched the network and the spanning tree in green, which is my BFS. Then I calculated the y's and finally the row zero coefficients for the NBV's (all are less than zero).
My problem is that I do not know if a NBV is lower or upper. In case of the maximum flow of 12, all NBV's are in between upper and lower limits. Are they then upper or lower NBV's, when I test if they are less or greater than zero?
I hope someone has an answer, because I am really desperate.