Missing Return Keyword in Solution to Exercise 2.11

[bpetermann]

In the solution to task 2.11, the return keyword is missing in the the_trouble_maker function, which leads to the Error: head(xs) expects a pair as argument xs, but encountered undefined

function the_trouble_maker(xl, xu, yl, yu) {
    const p1 = xl * yl;
    const p2 = xl * yu;
    const p3 = xu * yl;
    const p4 = xu * yu;
    // Missing return
   make_interval(math_min(p1, p2, p3, p4),
                  math_max(p1, p2, p3, p4));
}