```python
def trace_relay(links: dict, start: str) -> list:
    if not isinstance(links, dict): raise ValueError("links must be a mapping")
    if start not in links: raise ValueError("start is not a post")
    route = [start]
    post = start
    while True:
        next_post = links.get(post)
        if next_post is None or next_post == "":
            break
        if next_post in route:
            raise ValueError(f"watch comes round again at {next_post}")
        route.append(next_post)
        post = next_post
    return route
```