```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] = []
    start, end = minutes(span)
    for booking in sorted(booked, key=lambda x: minutes(x)[0]):
        b_start, b_end = minutes(booking)
        if b_start > start:
            free.append(stamp(start) + "-" + stamp(b_start))
        start = max(start, b_end)
    if start < end:
        free.append(stamp(start) + "-" + stamp(end))
    return ", ".join(free) if free else "none"
```