set NODES;   # nodeset of digraph D

param demand {NODES} ;  # amounts required at destinations,
			# negative for supply nodes, zero for other nodes

   check: sum {j in NODES} demand[j] == 0;

param cost {NODES,NODES}     ;   # shipment costs per unit, any real number
var Xvar   {NODES,NODES} >= 0;    # units to be shipped

minimize Total_Cost:
   sum {i in NODES, j in NODES} cost[i,j] * Xvar[i,j];

subject to Flowconserv {i in NODES}:
   sum {j in NODES} Xvar [j,i] - (sum {j in NODES} Xvar [i,j]) = demand[i];