```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}"

    span_minutes = minutes(span)
    booked_minutes = sorted(minutes(b) for b in booked)

    free: list[str] = []
    current_time = span_minutes[0]

    for start, end in booked_minutes:
        if start > current_time:
            free.append(stamp(current_time) + "-" + stamp(start))
        current_time = max(current_time, end)

    if current_time < span_minutes[1]:
        free.append(stamp(current_time) + "-" + stamp(span_minutes[1]))

    return ", ".join(free) if free else "none"
```