code: def Append(self, value, timestamp):
      
  
      timestamp = self._NormalizeTime(timestamp)
      if self.data and timestamp < self.data[-1][1]:
        raise RuntimeError("Next timestamp must be larger.")
      self.data.append([value, timestamp])
docs: Adds value at timestamp.
  
      Values must be added in order of increasing timestamp.
  
      Args:
        value: An observed value.
        timestamp: The timestamp at which value was observed.
  
      Raises:
        RuntimeError: If timestamp is smaller than the previous timstamp.