12c12
<     tail: *Elephant = undefined, // <---- make this optional!
---
>     tail: ?*Elephant = undefined,
39,42c39
<         // We should stop once we encounter a tail that
<         // does NOT point to another element. What can
<         // we put here to make that happen?
<         if (e.tail == null) ???;
---
>         if (e.tail == null) break;