To those who is not in the Hall-D.SVN.Repository mailing list
and is using genr8_hddm. I changed one parameter in the genr8_hddm:
option V"x y z_min z_max" allows one to distribute z coordinates
of the interaction vertices uniformly between z_min and z_max.
To produce an hddm file with the fixed z vertex, set z_min equal to
z_max.
Cheers,
Sascha