docs: def save(self, txt_fst_filename):
          """
          Save the machine in the openFST format in the file denoted by
          txt_fst_filename.
          Args:
              txt_fst_filename (str): The name of the file
          Returns:
              None
          """
          txt_fst = open(txt_fst_filename, 'w+')
          states = sorted(self.states, key=attrgetter('initial'), reverse=True)
          for state in states:
              for arc in state.arcs:
                  itext = self.isyms.find(arc.ilabel)
                  otext = self.osyms.find(arc.ilabel)
                  txt_fst.write(
                      '{}\t{}\t{}\t{}\n'.format(
                          state.stateid,
                          arc.nextstate,
                          itext.encode('hex'),
                          otext.encode('hex')))
              if state.final:
                  txt_fst.write('{}\n'.format(state.stateid))
          txt_fst.close()