pub fn parse_key_val_opt<T, U>( s: &str, ) -> Result<(T, Option<U>), Box<dyn Error + Send + Sync + 'static>>where T: FromStr, T::Err: Error + Send + Sync + 'static, U: FromStr, U::Err: Error + Send + Sync + 'static,
Parse a single key-value pair where value can be null