Function strict_real_elt 

fn strict_real_elt<T>(v: f64) -> Result<Option<T>, String>
where
    f64: TryCoerce<T>,
    <f64 as TryCoerce<T>>::Error: Display,

One REALSXP element: NA_real_ is None; any other value coerces, so a fractional, NaN or out-of-range double fails.