code: def SaveAs(self, filename):
          
          directory, zfile = _os.path.split(filename)
          if zfile.startswith('pyzos_ui_sync_file'):
              self._iopticalsystem.SaveAs(filename)
          else: # regular file
              if not _os.path.exists(directory):
                  raise ValueError('{} is not valid.'.format(directory))
              else:
                  self._file_to_save_on_Save = filename   # store to use in Save()
                  self._iopticalsystem.SaveAs(filename)
docs: Saves the current system to the specified file. 
  
          @param filename: absolute path (string)
          @return: None
          @raise: ValueError if path (excluding the zemax file name) is not valid
  
          All future calls to `Save()`  will use the same file.