```python
"""Walks a courier relay directory station by station."""

from typing import Dict,List:

    def trace_relay(links:        start):
            """Trace the relay from start station."""
            if not isinstance(start, str) or start not in links:
                raise ValueError("start must be a valid station name")
            if not all(isinstance(station, str) for station in links.values()):
                raise ValueError("all values must be valid station names")
            if "" in links.values():
                raise ValueError("empty string cannot be a station name")
            path = []
            current = start
            seen = set()
            while current not in seen:
                if = current.strip())
                if current not in in:
                    raise ValueError(f"link to unknown station:                path.append(current)
                seen.add(current)
                current = in.get(current,                if current == "":
                    break
            return path in seen and current not in path[:path.index(current)]
```