str::split and str::splitn doc unclear as to how empty string input is handled

[pnkfelix]

Here's the doc for str::split and str::splitn:

http://static.rust-lang.org/doc/master/core/str/trait.StrSlice.html#tymethod.split

Pop quiz: what does this program print. In particular, what will the first call to next() return when we are doing a split over the empty string? (Also, should it do something else? But that's not as important to me as making the doc clear.)

fn take_three<'a, I:Iterator<&'a str>>(input: &str, variant: &str, mut i: I) {
    let fst = i.next();
    let snd = if fst.is_some() { i.next() } else { None };
    let thd = if snd.is_some() { i.next() } else { None };
    println!("{:8} {:3}: {} {} {}",
             format!("`{}`", input), variant, fst, snd, thd);
}

fn main() {
    let i = "a.b";
    take_three(i,   "", i.split('.'));

    let i = ".";
    take_three(i,   "", i.split('.'));

    let i = "";
    take_three(i,   "", i.split('.'));

    let i = "a.b";
    take_three(i, "n1", i.splitn('.', 1));

    let i = ".";
    take_three(i, "n1", i.splitn('.', 1));

    let i = "";
    take_three(i, "n1", i.splitn('.', 1));

    let i = "a.b";
    take_three(i, "n0", i.splitn('.', 0));

    let i = ".";
    take_three(i, "n0", i.splitn('.', 0));

    let i = "";
    take_three(i, "n0", i.splitn('.', 0));
}

(To be fair: this is not one of those trick questions where splitn's behavior deviates from split on the empty string. If you have the correct intuition about how one behaves, then you'll have the right intuition for the other. But I didn't have an intuition in either direction, which led me to first consult the doc.)