Chapter 17.2: undescribed repetition pattern?

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?

Start by reading chapter 17.2 around the assert_equal_len! macro and its surrounding explanation. Clarify the purpose of the parenthesized expressions with trailing commas, and consider the issue done when a reader can understand that syntax from the chapter text.