def get_tid2synonyms(self, clean:bool=True):
        ''' clean: for list of literals only '''
        tid2synonyms = {}
        header = ['Index'] + list(self.fetch_synonyms().columns)
        for row in self.fetch_synonyms().itertuples():
            row = {header[i]:val for i, val in enumerate(row)}
            if clean:
                synonym = {'literal':row['literal'], 'type':row['type']}
                tid2synonyms[row['tid']].append(synonym)
            elif not clean:
                tid2synonyms[row['tid']].append(row)
        return tid2synonyms
clean: for list of literals only