Macro optgroup 

macro_rules! optgroup {
    ($($all:tt)*) => { ... };
}

Example

use rtml::*;

assert_eq!(
    optgroup![.label="Fruits", option!["Apple"], option!["Banana"], option!["Cherry"]].render(),
    "<optgroup label=\"Fruits\"><option>Apple</option><option>Banana</option><option>Cherry</option></optgroup>"
);