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"]

test "corecursive functions"
    >> ping(3)
    assert ping(3) == ["ping: 3", "pong: 2", "ping: 1", "pong: 0"]