The Trail and Lift Selection section of the event graph is the real engine behind the model; skiers enter this section having arrived at a trailhead and are routed, based on the architecture of the resort, to their next trail or lift. Skiers are sent to Probability Calc from lifts (the arrow entering Probability Calc from the bottom comes from the Unload Lift event), the ski resort Base (the arrow entering Probability Calc from the left comes from the Base event, or from other trails (the arrow from Trailhead to Probability Calc). The Probability Calc event checks the probability that the skier will go from the current trailhead to every other trailhead and recursively calls itself until it finds where the skier will go next. Once the destination trailhead is found, Probability Calc calls the Trailhead event with the origin and destination trailhead. If the destination trailhead is the base, the skier is sent by the arrow leaving the Trailhead event to the left to the Base event after the amount of time that it takes to get from the origin trailhead to the base. If the destination trailhead is a lift, the skier is sent by the arrow leaving downwards from Trailhead event to the Queue event after the amount of time that it takes to get from the origin trailhead to the selected lift. If the destination trailhead is top of another trail, the skier is sent from Trailhead event back to the Probability Calc event after the amount of time that it takes to get from the origin trailhead to the destination trailhead.