pub fn as_extension_from_base<AB: SP1AirBuilder<Var = T>>(
        &self,
        base: AB::Expr,
    ) -> SepticExtension<AB::Expr> {
        let mut arr: [AB::Expr; 7] = self.0.clone().map(|_| AB::Expr::zero());
        arr[0] = base;

        SepticExtension(arr)
    }
Takes a single expression into a `SepticExtension` of expressions.
crates/stark/src/septic_extension.rs