code: def _sumterm_prime(lexer):
      
      tok = next(lexer)
      # '|' XORTERM SUMTERM'
      if isinstance(tok, OP_or):
          xorterm = _xorterm(lexer)
          sumterm_prime = _sumterm_prime(lexer)
          if sumterm_prime is None:
              return xorterm
          else:
              return ('or', xorterm, sumterm_prime)
      # null
      else:
          lexer.unpop_token(tok)
          return None
docs: Return a sum term' expression, eliminates left recursion.