Function realize 

pub fn realize<'a>(
    kind: RealizationKind<'_>,
    engine: &mut Engine<'_>,
    locator: &mut SplitLocator<'_>,
    arenas: &'a Arenas,
    content: &'a Content,
    styles: StyleChain<'a>,
) -> SourceResult<Vec<Pair<'a>>>

Realize content into a flat list of well-known, styled items.