Lecture 4.4 - Variance (Optional) (21:33)
In video quiz
17:58 / 21:33

Implement prepend as shown in trait List.
def prepend[U >: T](elem: U): List[U] = new Cons(elem, this)

What is the result type of this function:
def f(xs: List[NonEmpty], x: Empty) = xs prepend x