Vietnamese literature has centuries-deep history and the country has a rich tradition of folk literature based on the typical six–to-eight-verse poetic form called ''ca dao'' which usually focuses on village ancestors and heroes. Written literature has been found dating back to the 10th century Ngô dynasty, with notable ancient authors including: Nguyễn Trãi, Trần Hưng Đạo, Nguyễn Du and Nguyễn Đình Chiểu. Some literary genres play an important role in theatrical performance, such as ''hát nói'' in ''ca trù''. Some poetic unions have also been formed in Vietnam, such as the ''tao đàn''. Vietnamese literature has been influenced by Western styles in recent times, with the first literary transformation movement of ''thơ mới'' emerging in 1932. Vietnamese folk literature is an intermingling of many forms. It is not only an oral tradition, but a mixing of three media: hidden (only retained in the memory of folk authors), fixed (written), and shown (performed). Folk literature usually exists in many versions, passed down orally, and has unknown authors. Myths consist of stories about supernatural beings, heroes, creator gods and reflect the viewpoint of ancient people about human life. They consist of creation stories, stories about their origins (''Lạc Long Quân'' and ''Âu Cơ''), culture heroes (''Sơn Tinh'' and ''Thủy Tinh'') which are referred to as a mountain and water spirit respectively and many other folklore tales.