Dear Herman,
The sketch found here may help, particularly the reference to John Makeham's book,
which will probably be the most promising lead to follow:
https://en.wikipedia.org/wiki/Xu_Gan
best,
Matthew
Matthew Kapstein
Directeur
d'études,
Ecole Pratique des Hautes Etudes
Numata Visiting Professor of Buddhist Studies,
The University of Chicago