Chapter 17.2: undescribed repetition pattern?

[Feliksas]

Code example in chapter 17.2 has the following macro defined:

macro_rules! assert_equal_len {
    // The `tt` (token tree) designator is used for
    // operators and tokens.
    ($a:expr, $b:expr, $func:ident, $op:tt) => {
        assert!($a.len() == $b.len(),
                "{:?}: dimension mismatch: {:?} {:?} {:?}",
                stringify!($func),
                ($a.len(),),
                stringify!($op),
                ($b.len(),));
    };
}

It is unclear why $a.len() and $b.len() are wrapped in parentheses with a trailing comma inside after the expression - looks like it could mean multiple instances (or lack) of an expression, but the context does not seem to allow for that. What is the purpose of that syntax?