 In his (1986c: 200), Lewis distinguishes cases of early and late preemption. In early preemption examples, the process running from the preempted alternative is cut short before the main process running from the preempting cause has gone to completion. The example of the two assassins, given above, is an example of this sort. The theory of causation in terms of chains of causal dependence can handle this sort of example. In contrast, cases of late preemption are ones in which the process running from the preempted cause is cut short by the main process running to completion and bringing about the effect before the preempted potential cause has the opportunity to do so. The following is an example of late preemption due to Hall (2004).