Repetitions in macros

Apart from token tree types, we also need a way to repeatedly generate certain parts of our code. One of the practical examples from the standard library is the vec![] macro, which relies on repetition to give an illusion of variadic arguments, and allows you to create Vecs in any of the following manners:

vec![1, 2, 3];
vec![9, 8, 7, 6, 5, 4];

Let's see how vec! does this. Here's vec's macro_rules! definition from the standard library:

macro_rules! vec {
    ($elem:expr; $n:expr) => (
        $crate::vec::from_elem($elem, $n)
    );
    ($($x:expr),*) => (
        <[_]>::into_vec(box [$($x),*])
    );
    ($($x:expr,)*) => (vec![$($x),*])
}

By ignoring the details to the right of => and focusing on the last two matching rules on the left-hand side, we can see something new in these rules:

($($x:expr),*)
($($x:expr,)*)

These are repeating...