```ts

export function traceRelay(links: Record<string, string>, start: string): string[] {
  if (links === null || typeof links !== "object" || Array.isArray(links)) throw new Error("links must be a mapping");
  if (!(start in links)) throw new Error("start is not a post");

  const route: string[] = [];
  let currentPost = start;
  const visitedPosts = new Set<string>();

  while (currentPost !== "") {
    if (!visitedPosts.has(currentPost)) {
      visitedPosts.add(currentPost);
      route.push(currentPost);

      if (!links.hasOwnProperty(currentPost)) {
        throw new Error("handoff naming a post that links does not name");
      }

      currentPost = links[currentPost];
    } else {
      throw new Error("a run of handoffs that comes back to a post already visited");
    }
  }

  return route;
}
```