Consider a transhipment context for the minimum cost flow problem where the problem is to optimally transfer flow from some supply points to some demand points over arcs of limited capacity. In a dynamic version of this context, the transfer is to be performed over N time units, and transferring flow along an arc (i, j) requires time τij , which is a given positive integer number of time units. This means that at each time t = 0,...,N - τij , we may send from node i along arc (i, j) a flow xij ∈ [0, cij ], which will arrive at node j at time t+τij . Formulate this problem as a minimum cost flow problem involving a copy of the given graph for each time period.