Function arg_combinators::opt_default

pub fn opt_default<T>(
    short: &str, 
    long: &str, 
    doc: &str, 
    hint: &str, 
    default: T
) -> ArgExt<impl Arg<Item = T>> where
    T: Clone + FromStr + Debug + Display,
    <T as FromStr>::Err: Clone + Debug + Display,