in book/master_middleman/source/javascripts/waypoints/group.js [60:65]
Group.prototype.next = function(waypoint) {
this.waypoints.sort(byTriggerPoint)
var index = Waypoint.Adapter.inArray(waypoint, this.waypoints)
var isLast = index === this.waypoints.length - 1
return isLast ? null : this.waypoints[index + 1]
}