Function render_frame_part_with_prose 

pub fn render_frame_part_with_prose<F>(
    book: &BridgeBook,
    part: &BridgePart,
    render_prose: F,
) -> Result<String>
where
    F: FnMut(&Symbol, &Expr) -> Result<String>,

Renders a bridge/Frame part as a cited fluent sentence, delegating prose holes to render_prose.