n = 4;
m = 6;

demand = [-10.0, 0.0, 0.0, 10.0];
arcs = [|1, 2 
        |1, 3 
        |1, 4 
        |2, 3 
        |2, 4 
        |3, 4|];

capacity = [ 6.0, 4.0, 4.0,
             3.0, 3.0,
             3.0];

capacity_lb = [ 0.0, 0.0, 0.0,
                0.0, 0.0,
                0.0,];

costs = [ 10.0, 6.0, 10.0,
           2.0, 10.0,
           3.0];