Method set_explain

src/script.rs:82–88

(&mut self, explain: &str)

            }
            None => PreBgm::Stop,
        };
        let pre_bg = self
            .backgrounds
            .range(..=index)
            .next_back()
            .map(|(_, bg)| bg.clone());
        let pre_figures = self
            .figures
            .range(..=index)
            .next_back()