```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");
  }
  const singular = String(Math.abs(amount)) + " " + unit;
  return singular.charAt(singular.length - 1) === '1' ? singular : singular + 's';
}
```