docs: def current_heart_rate(self):
          """Return current heart rate for in-progress session."""
          try:
              rates = self.intervals[0]['timeseries']['heartRate']
              num_rates = len(rates)
  
              if num_rates == 0:
                  return None
  
              rate = rates[num_rates-1][1]
          except KeyError:
              rate = None
          return rate