> “f: list[1 A] -> list[A] pure, worst-case time n log n, such that for all x and 0 <= i <= j < len(x), f(x)[i] <= f(x)[j]” is probably good enough for nearly everyone

Not good enough: `f(x) = []` or `f(x) = (if len(x) == 0 then [] else [x[0], x[0]]` are implementations that fulfill your specification and yet they don't always sort the input list correctly...