Go でゴルーチンの実行順序を制御するにはどうすればよいですか?

func sum(a []int, c chan int) {
    fmt.Println("summing: ", a)
    total := 0
    for _, v := range a {
        total += v
    }
    c <- total  // send total to c
}

func main() {
    c := make(chan int)

    go sum([]int{1,2,3}, c)
    go sum([]int{4,5,6}, c)

    x := <-c
    fmt.Println(x)
    x = <-c
    fmt.Println(x)
}

summing:  [4 5 6]
15
summing:  [1 2 3]
6

summing:  [1 2 3]
6
summing:  [4 5 6]
15

func main() {

    c := make(chan int)

    go sum([]int{1, 2, 3}, c)

    // Blocks until a value is received
    x := <-c
    fmt.Println(x)

    // Execute the next goroutine
    go sum([]int{4, 5, 6}, c)

    x = <-c
    fmt.Println(x)
}

func sum(a []int, c chan int, wg *sync.WaitGroup) {
    defer wg.Done()
    fmt.Println("summing: ", a)
    total := 0
    for _, v := range a {
        total += v
    }
    c <- total // send total to c
}

func main() {

    c := make(chan int)
    wg := new(sync.WaitGroup)

    // Increment the wait group
    wg.Add(1)
    // Launch the first goroutine
    go sum([]int{1, 2, 3}, c, wg)

    // Wait for the first goroutine to complete
    wg.Wait()

    // Increment the wait group again
    wg.Add(1)
    // Launch the second goroutine
    go sum([]int{4, 5, 6}, c, wg)

    // Wait for the second goroutine to complete
    wg.Wait()

    // Close the channel to indicate that no more values will be sent
    close(c)

    // Range over the channel to receive the results
    for theSum := range c {
        x := theSum
        fmt.Println(x)
    }

}