```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 = unit[:-1] if unit.endswith('s') else unit
    return str(amount) + " " + singular + "s"
```