```python
"""Report the stretches of a clinic's day that no appointment covers."""

import re


def open_windows(span: str, booked: list[str]) -> str:
    def minutes(text: str) -> list[int]:
        parts = text.split("-")
        if len(parts) != 2 or any(re.fullmatch(r"\d{2}:\d{2}", p) is None for p in parts):
            raise ValueError("a range must be written HH:MM-HH:MM: " + text)
        return [int(p[:2]) * 60 + int(p[3:]) for p in parts]

    def stamp(at: int) -> str:
        return f"{at // 60:02d}:{at % 60:02d}"

    free: list[str] = []
    # TODO: sweep the appointments in time order and keep what the span has left over.
    for start, end in sorted(minutes(span)):
        for book_start, book_end in sorted(minutes(booked)):
            if start < book_end and end > book_start:
                start = max(start, book_end)
            elif start > book_end:
                free.append(stamp(start))
                start = book_end
        free.append(stamp(start))
    if free:
        return ", ".join(free)
    else:
        return "none"
```