set D_CITY;
set W_CITY;
set DW_LINKS within (D_CITY cross W_CITY);

param p_supply >= 0;           # amount available at plant
param w_demand {W_CITY} >= 0;  # amounts required at warehouses

   check: p_supply = sum {j in W_CITY} w_demand[j];

param pd_cost {D_CITY} >= 0;   # shipment costs/1000 packages
param dw_cost {DW_LINKS} >= 0;

param pd_cap {D_CITY} >= 0;    # max packages that can be shipped
param dw_cap {DW_LINKS} >= 0;

minimize Total_Cost;

node Plant: net_out = p_supply;

node Dist {i in D_CITY};

node Whse {j in W_CITY}: net_in = w_demand[j];

arc PD_Ship {i in D_CITY} >= 0, <= pd_cap[i],
   from Plant, to Dist[i], obj Total_Cost pd_cost[i];

arc DW_Ship {(i,j) in DW_LINKS} >= 0, <= dw_cap[i,j],
   from Dist[i], to Whse[j], obj Total_Cost dw_cost[i,j];