```python
def short_unit(unit: str) -> str:
    return unit[:3]


def label_of(amount: int, unit: str) -> str:
    """An amount written with its unit."""
    if amount < 0:
        raise ValueError("amount cannot be negative")
    singular = f"{amount} {unit}"
    plural = singular.replace(unit, unit + "s", 1)
    return singular if amount == 1 else plural
```