old_contents: import libvtd.trusted_system
  
  
  def UpdateTrustedSystem(file_name):
      """Make sure the TrustedSystem object is up to date."""
      global my_system
      my_system = libvtd.trusted_system.TrustedSystem()
      my_system.AddFile(file_name)
  
new_contents: import libvtd.trusted_system
  
  
  def UpdateTrustedSystem(file_name):
      """Make sure the TrustedSystem object is up to date."""
      global my_system
      if 'my_system' not in globals():
          my_system = libvtd.trusted_system.TrustedSystem()
          my_system.AddFile(file_name)
      my_system.Refresh()
  
subject: Refresh system instead of clobbering it