Method set_pre_bg

src/script.rs:113–115

(&mut self, pre_bg: Option<Command>)

    read_block: usize,
    current_bgm: String,
    pre_voice: Option<(SharedString, SharedString)>,
    timeline: Timeline,
    clear: HashSet<usize>,
    labels: HashMap<String, usize>,
    pre_items: PreItems,
}