```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]
    visited = set([start])
    current_post = start

    while True:
        next_post = links.get(current_post)
        if next_post is None or next_post == "":
            break
        if next_post in visited:
            raise ValueError("watch comes back to a post already visited")
        route.append(next_post)
        visited.add(next_post)
        current_post = next_post

    return route
```