n = 5;
m = 10;

demand = [-10.0, 0.0, 0.0, 0.0, 10.0];
arcs = [|1, 2 
        |1, 3 
        |1, 4 
        |1, 5 
        |2, 3 
        |2, 4 
        |2, 5 
        |3, 4 
        |3, 5 
        |4, 5|];

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

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

costs = [ 10.0, 6.0, 10.0, 20.0,
              2.0,  4.0, 10.0,
                  2.0, 10.0,
                      2.0];