  Manuscript fragments discovered in the Dunhuang caves form another important source in Laozi research. Among them are several Heshanggong fragments (especially S. 477 and S. 3926 in the Stein collection, and P. 2639 in the Pelliot collection) and the important Xiang’er Laozi with commentary. Another Dunhuang manuscript that merits attention is the “Suo Dan” fragment, now at the University Art Museum, Princeton University, which contains the last thirty-one chapters of the Daodejing beginning with chapter 51 of the modern text. It is signed and dated at the end, bearing the name of the third-century scholar and diviner Suo Dan, who is said to have made the copy, written in ink on paper, in 270 C.E. According to Rao Zongyi (1955), the Suo Dan version belongs to the Heshanggong line of the Laozi text. William Boltz (1996), however, questions its third-century date and argues that the fragment in many instances also agrees with the Fu Yi “ancient version.”