Original code from V. Drollinger, see notes
CMS IN 2005/004
For GENMOD = 2, need a link called "input.txt" which must point to :
input.txt should be in the directory where you run cmsRun
Added on February 25:
to run with the MARS files, you need a link for bgas.123 and tails.1,
in the directory where you run cmsRun.
The files are in /afs/cern.ch/cms/beam_halo/.