@app.get("/items/{item_id}")
def main(
    item_id: int = Path(default=..., gt=0),
    query: str = Query(default=..., max_length=10),
    session: str = Cookie(default=..., min_length=3),
    x_trace: str = Header(default=..., title="Tracing header"),
):
    return {"message": "Hello World"}
```

...all these parameters are required because the default value is `...` (Ellipsis).

a single one that can proceed is chosen via a uniform pseudo-random selection.
Otherwise, if there is a default case, that case is chosen.
If there is no default case, the "select" statement blocks until
at least one of the communications can proceed.
</li>

<li>
Unless the selected case is the default case, the respective communication
operation is executed.
</li>

<li>