```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;
  const plural = singular + "s";
  return Math.abs(amount) === 1 ? singular : plural;
}
```