I want to know the answer to the following question:
The Shu were an ally of who, according to the Book of Documents?
All the background I'm provided with is that "Accounts of Shu exist mainly as a mixture of mythological stories and historical legends recorded in local annals such as the Chronicles of Huayang compiled in the Jin dynasty (265–420), with folk stories such as that of Emperor Duyu (杜宇) who taught the people agriculture and transformed himself into a cuckoo after his death.". Is that enough to answer the question?