```ts
/** Report the stretches of a clinic's day that no appointment covers. */
export function openWindows(span: string, booked: string[]): string {
  const clock = /^\d{2}:\d{2}$/;
  const minutes = (range: string): number[] => {
    const parts = range.split("-");
    if (parts.length !== 2 || !clock.test(parts[0]) || !clock.test(parts[1])) {
      throw new Error("a range must be written HH:MM-HH:MM: " + range);
    }
    return parts.map((at) => Number(at.slice(0, 2)) * 60 + Number(at.slice(3)));
  };
  const stamp = (at: number): string =>
    String(Math.floor(at / 60)).padStart(2, "0") + ":" + String(at % 60).padStart(2, "0");
  const free: string[] = [];
  let start = minutes(span.split("-")[0]);
  let end = minutes(span.split("-")[1]);

  for (const ap of booked) {
    const apStart = minutes(ap.split("-")[0]);
    const apEnd = minutes(ap.split("-")[1]);

    if (apStart < start) {
      free.push(stamp(start));
    } else if (apStart <= end) {
      start = Math.max(start, apEnd);
    }
  }

  if (start < end) {
    free.push(stamp(end));
  }

  return free.length === 0 ? "none" : free.join(", ");
}
```