```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 = []
    post = start
    while post != "":
        if post not in links: raise ValueError("unknown post")
        route.append(post)
        post = links[post]
    return route
```