Function write_future_option_vol 

fn write_future_option_vol(
    f: &mut Formatter<'_>,
    contract: &ListedFutureId,
    expiry: Option<NaiveDate>,
    strike_bp: Option<u32>,
) -> Result

A futures option vol node’s label: "future option vol CONTRACT", then the listed expiry and @ STRIKE where the surface carries them.