```python
"""Walks a courier relay directory station by station."""
def trace_relay(links, start):
    if not isinstance(start, str) or start not in links:
        raise ValueError("unknown starting station")
    path = []
    current = start
    for _ in range(len(links)):
        path.append(current)
        target = links[current]
        if target == "" or target not in links:
            return path
        current = target
    return path
```