```ts
/** Work stretches of a shift: span-long, rest-separated, short tails dropped. */
export function carveShift(
  start: number,
  end: number,
  span: number,
  rest: number,
  least: number,
): number[][] {
  for (const value of [start, end, span, rest, least]) {
    if (!Number.isInteger(value)) {
      throw new Error("every argument must be an integer");
    }
  }
  if (start >= end) {
    throw new Error("shift start must precede its end");
  }
  if (span < 1 || rest < 1) {
    throw new Error("span and rest must be at least 1");
  }
  if (least < 1 || least > span) {
    throw new Error("least must be between 1 and span");
  }

  const stretches: number[][] = [];
  for (let current = start; current + span < end;) {
    const to = Math.min(current + span, end);
    current += to - current;
    if (to - current >= least) {
      stretches.push([current, to]);
      current += rest;
    }
  }
  return stretches;
}
```