```ts
export function shortUnit(unit: string): string {
  return unit.slice(0, 3);
}

/** An amount written with its unit. */
export function labelOf(amount: number, unit: string): string {
  if (amount < 0) {
    throw new Error("amount cannot be negative");
  } else if (amount === 1 && unit !== "one") {
    return String(amount) + " " + unit;
  } else {
    return String(amount) + " " + unit + "s";
  }
}
```