```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] = []
    span_start, span_end = minutes(span)
    booked.sort(key=minutes)

    if not all(start < end for start, end in booked):
        raise ValueError("appointments must be written HH:MM-HH:MM and non-overlapping")

    last_end = span_start
    for start, end in booked:
        if start > last_end:
            free.append(f"{stamp(last_end)}-{stamp(start)}")
        last_end = max(last_end, end)

    if last_end < span_end:
        free.append(f"{stamp(last_end)}-{stamp(span_end)}")

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