Why is this C++ fold expression valid?

Question

On cppreference, I saw that there are four types of fold expressions, unary right, unary left, binary right, and binary left. What is the type of this fold expression here? I'm having a hard time understanding why it is valid.

    template <typename Res, typename... Ts>
    vector<Res> to_vector(Ts&&... ts) {
        vector<Res> vec;
        (vec.push_back(ts) ...); // *
        return vec;
    }

What is the value of "pack", "op" and "init" in line *, if any?

This example is from page 244 of Bjarne Stroustrup's A Tour of C++ book, and seems like a comma was forgotten in the example, hence my confusion.

Answer 1

The syntax is not valid. It's missing a comma (most likely a typo):

(vec.push_back(ts), ...)
//                ^

And so it is "unary right fold":

( pack op ... )

with op being a comma.