    /// "Invisible" mark, used in `Annotation` to ensure that a span is visible
    /// within the snippet.  Does _not_ add any visual indicators to the
    /// output, and does not accept a label.
    Show,

    /// Low-priority visual marking.
    Mark(Option<LabelStr>),
    
    /// High-priority visual marking, used to call attention to the marked span
    /// using a distinctive visual style.
    Highlight(Option<LabelStr>)
}
