```python
"""A cleaned form of a slash-separated relative path."""

def normalize_rel_path(path: str) -> str:
    if not isinstance(path, str):
        raise ValueError("Input must be a string")
    if path == "" or path.startswith("/"):
        raise ValueError("Invalid path")

    out = []
    for segment in path.split("/"):
        if segment == "..":
            if out:
                out.pop()
            else:
                raise ValueError("Path would climb above the starting point")
        elif segment != ".":
            out.append(segment)

    return "/".join(out) or "."
```