```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)
    for booked_start, booked_end in sorted(booked, key=lambda x: minutes(x)):
        if booked_start <= span_start:
            span_start = booked_end + 1
        elif booked_end < span_end:
            free.append(stamp(span_start) + "-" + stamp(booked_end))
            span_start = booked_end + 1
    if span_start <= span_end:
        free.append(stamp(span_start) + "-" + stamp(span_end))
    return ", ".join(free) if free else "none"
```