[Kwant] Smatrix in hall bar

Hello all, I have a question about one of the functions in this link: https://nbviewer.jupyter.org/url/topocondmat.org/notebooks/w3_pump_QHE/Laugh... For plotting Hall conductance and longitudinal one, there is a section where you defined a G function, which I put it below: def G(syst, p): smatrix = kwant.smatrix(syst, args=[p]) G = [[smatrix.transmission(i, j) for i in range(num_leads)] for j in range(num_leads)] G -= np.diag(np.sum(G, axis=0)) return calculate_sigmas(G) I want to know why we need this line " G -= np.diag(np.sum(G, axis=0)) " since G matrix is already calculated in the previous line? Your time and consideration are greatly appreciated. Best regards, Ali