```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("Path cannot be empty or start with a slash.")
    
    segments = path.split("/")
    stack = []
    
    for segment in segments:
        if segment == "..":
            if not stack:
                raise ValueError("Cannot climb above the starting point.")
            stack.pop()
        elif segment != ".":
            stack.append(segment)
    
    return "/" + "/".join(stack) if stack else "."
```