What does it mean to define X1 and X2 directly? Well, X1 needs to be a function that
-takes a number as argument n and returns true if n is even and false if n is not even.
+takes a number `n` as argument and returns `true` if `n` is even and `false` if `n` is not even.
So X1 must have the behavior of the `even` function. Fortunately, it's easy to write
such a function without using any fixed-point combinators...