code: def add_lfn(self,lfn):
      
      if len(self.table['process']['stream']) > 1:
        msg = "cannot add lfn to table with more than one process"
        raise LIGOLwParseError, msg
      # get the process_id from the process table
      pid_col = self.table['process']['orderedcol'].index('process_id')
      pid = self.table['process']['stream'][0][pid_col]
      try:
        self.table['lfn']['stream'].append((pid,lfn))
      except KeyError:
        self.table['lfn'] = {
          'pos' : 0,
          'column' : {'process_id' : 'ilwd:char', 'name' : 'lstring'},
          'stream' : [(pid, lfn)],
          'query' : '',
          'orderedcol' : ['process_id', 'name' ]
          }
docs: Add an LFN table to a parsed LIGO_LW XML document.
  
      lfn = lfn to be added