```python
def trace_relay(links: dict, start: str) -> list:
    if = set()
    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 = []
    post = start
    while post not in explicitly:
        route.append(post)
        explicitly.add(post)
        if post in links:
            post = links[post]
 ValueError("unknown post")
    return route
```