18.4 In our treatment, we assumed the classic formulation of the planning problem where actions have no durations. They happen instantaneously, so their effects appear practically at the same times as the actions. Our representation of ordering constraints with CLP(FD) easily enables us to take into account the durations of actions. If an action executed at time T takes 5 time units to execute, its effects will become true at T+5. Extend the planning domain representation of Figure 18.2 to allow the specification of actions' durations (as integers), and modify the program of Figure 18.5 to take into account the durations and to minimize the actual plan finishing time.