Function flatten_with 

pub fn flatten_with(
    text: &Text,
    base_weight: FontWeight,
    theme: &Theme,
    paint: impl Fn(&str) -> Option<MarkPaint>,
    preview: impl Fn(&str) -> Option<Preview>,
) -> Flat

flatten with the app’s own marks painted — see crate::MarkPaint — and each mention described by preview. A name the app does not paint reads as the text it wraps.