func ping(x:Int->[Text])
	if x > 0
		return ["ping: $x"] ++ pong(x-1)
	else
		return ["ping: $x"]

func pong(x:Int->[Text])
	if x > 0
		return ["pong: $x"] ++ ping(x-1)
	else
		return ["pong: $x"]

func main()
	assert ping(3) == ["ping: 3", "pong: 2", "ping: 1", "pong: 0"]