code: def previous_minute(self, dt):
          
  
          idx = previous_divider_idx(self._trading_minutes_nanos, dt.value)
          return self.all_minutes[idx]
docs: Given a dt, return the previous exchange minute.
  
          Raises KeyError if the given timestamp is not an exchange minute.
  
          Parameters
          ----------
          dt: pd.Timestamp
              The dt for which to get the previous exchange minute.
  
          Returns
          -------
          pd.Timestamp
              The previous exchange minute.