predicate fzn_network_flow(array[int,1..2] of int: arc, 
	  	                 array[int] of int: balance,
                       array[int] of var int: flow) = 

    let { int: source_node = 1;
          int: sink_node = 2;
          set of int: ARCS = index_set_1of2(arc);
          set of int: NODES = index_set(balance);
    } in
	   forall (i in NODES) (
	      sum (j in ARCS where i == arc[j,source_node]) (flow[j]) - 
        sum (j in ARCS where i == arc[j,sink_node]) (flow[j])
        = balance[i] 
     );