What theories of the New Yuefu Movement did Bai Juyi and Yuan Zhen elaborate in their letters and poems?

Bai Juyi and Yuan Zhen expounded the theory of the New Yuefu Movement in their letters and poems respectively. There are mainly the following points: "Articles should be written in time, songs and poems should be written in content" (Bai Juyi's Nine Books with the Same Yuan) as the basic purpose of creation, advocating that poetry should reflect the times and social reality; Taking "helping the people to make up for the shortcomings of the times", "evaluating the current politics" and "venting human feelings" (Bai Juyi's Nine Books Yuyuan Garden) as the creative purpose, it is emphasized that poetry should serve politics; It is pointed out that poetry should express people's life with the content of "only songs make people sick" (Bai Juyi's "Sending") and "only hurting people's diseases" (Bai Juyi's "Wounding Tang Qu"). At the same time, they expounded the characteristics of poetry, pointing out that "those who touch people's hearts should not put feelings first, start with words, start with sound, and take meaning as deep" (Bai Juyi's Nine Books on Yuyuan Garden), and demanded that the content (affection) and form (sound) of poetry should be unified, and their Yuefu poems, such as selling charcoal Weng (Bai Juyi), should be "quality but diameter".