code: def _getShiftedGroundTruth(self, groundTruth):
      
  
      # Save this ground truth into our input history
      self._groundTruthHistory.append(groundTruth)
  
      # This is only supported when _predictionSteps has one item in it
      assert (len(self._predictionSteps) == 1)
      # Return the one from N steps ago
      if len(self._groundTruthHistory) > self._predictionSteps[0]:
        return self._groundTruthHistory.popleft()
      else:
        if hasattr(groundTruth, '__iter__'):
          return [None] * len(groundTruth)
        else:
          return None
docs: Utility function that saves the passed in groundTruth into a local
      history buffer, and returns the groundTruth from self._predictionSteps ago,
      where self._predictionSteps is defined by the 'steps' parameter.
      This can be called from the beginning of a derived class's addInstance()
      before it passes groundTruth and prediction onto accumulate().