```
sl.trackingshot.waypoints <-
function(projection, ...) {
waypoints = list(...)
for(a in waypoints){
if(!is.list(a) || class(a) != "sl.waypoint") stop("Arguments must be list and of class 'sl.waypoint'")
if(a$projection != projection) stop("Waypoints must have equal projection type")
if(!is.numeric(a$time)) stop("time of waypoint has to be numeric")
if(length(a$time) > 1) a$time = a$time[1]
if(a$time < 0) stop("time has to be a positive number")
a$projection = NULL
}
class(waypoints) <- "sl.waypoints"
waypoints$projection = projection
return(waypoints)
}
```

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