code: def get_freq(self):
          
          if not self.is_monotonic or not self.index._is_unique:
              return None
  
          delta = self.deltas[0]
          if _is_multiple(delta, _ONE_DAY):
              return self._infer_daily_rule()
  
          # Business hourly, maybe. 17: one day / 65: one weekend
          if self.hour_deltas in ([1, 17], [1, 65], [1, 17, 65]):
              return 'BH'
          # Possibly intraday frequency.  Here we use the
          # original .asi8 values as the modified values
          # will not work around DST transitions.  See #8772
          elif not self.is_unique_asi8:
              return None
  
          delta = self.deltas_asi8[0]
          if _is_multiple(delta, _ONE_HOUR):
              # Hours
              return _maybe_add_count('H', delta / _ONE_HOUR)
          elif _is_multiple(delta, _ONE_MINUTE):
              # Minutes
              return _maybe_add_count('T', delta / _ONE_MINUTE)
          elif _is_multiple(delta, _ONE_SECOND):
              # Seconds
              return _maybe_add_count('S', delta / _ONE_SECOND)
          elif _is_multiple(delta, _ONE_MILLI):
              # Milliseconds
              return _maybe_add_count('L', delta / _ONE_MILLI)
          elif _is_multiple(delta, _ONE_MICRO):
              # Microseconds
              return _maybe_add_count('U', delta / _ONE_MICRO)
          else:
              # Nanoseconds
              return _maybe_add_count('N', delta)
docs: Find the appropriate frequency string to describe the inferred
          frequency of self.values
  
          Returns
          -------
          str or None