```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
    plural = "s" if singular != " " + unit else ""
    return str(abs(amount)) + singular + plural
```