Baike says that it's a reference to 巴东三峡巫峡长，猿鸣三声泪沾裳, which is apparently a folk ballad.

Also that Du Fu quits drinking in his old age due to lung disease. How do we know that so specifically still, wow.

I enjoy the 394872374 variants of 'unfiltered rice wine' that exist. How are they different? No more easily described than the differences between grape wines, it seems.