```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 post != "":
        if post not in links:
            raise ValueError("handoff naming a post that links does not name")
        
        next_post = links[post]
        if next_post in route:
            raise ValueError("a watch that comes round again")
        
        route.append(next_post)
        post = next_post
    
    return route
```