pub fn as_extension<AB: SepticExtensionAirBuilder<Var = T>>(
        &self,
    ) -> SepticExtension<AB::Expr> {
        let arr: [AB::Expr; 7] = self.0.clone().map(|x| AB::Expr::zero() + x);
        SepticExtension(arr)
    }
Takes a `SepticBlock` into a `SepticExtension` of expressions.
crates/stark/src/septic_extension.rs