The next step in our example (after creating an empty Model
object) is to add variables to the model. The variables are created
using addVars, and are returned in a tupledict which
we'll call flow:
m = Model('netflow')
flow = m.addVars(commodities, arcs, obj=cost, name="flow")
The first, positional arguments to addVars give the index set.
In this case, we'll be indexing flow by commodities and
arcs. In other words, flow[c,i,j] will capture the flow
of commodity c from node i to node j. Note that
flow only contains variables for source, destination pairs
that are present in arcs.