Using a WaitGroup variable to manage concurrency

After appending thisCar to the generatedCars collection, we execute waitGroup.Done(). This decrements the count of the WaitGroup variable. This count corresponds to the iterator value we assign to the lower variable, and applies to the lower > upper expression that is assigned to the done return variable:

func carGenerator(iterator func(int) int, lower int, upper int) func() (int, bool) {
       return func() (int, bool) {
              lower = iterator(lower)
              return lower, lower > upper
       }
}

We use the iterator to know how many Goroutines to launch:

func iterator(i int) int {
       i += 1
       return i
}