code: def holtWintersForecast(requestContext, seriesList):
      
      previewSeconds = 7 * 86400  # 7 days
      # ignore original data and pull new, including our preview
      newContext = requestContext.copy()
      newContext['startTime'] = (requestContext['startTime'] -
                                 timedelta(seconds=previewSeconds))
      previewList = evaluateTokens(newContext, requestContext['args'][0])
      results = []
      for series in previewList:
          analysis = holtWintersAnalysis(series)
          predictions = analysis['predictions']
          windowPoints = previewSeconds // predictions.step
          result = TimeSeries("holtWintersForecast(%s)" % series.name,
                              predictions.start + previewSeconds,
                              predictions.end, predictions.step,
                              predictions[windowPoints:])
          result.pathExpression = result.name
          results.append(result)
      return results
docs: Performs a Holt-Winters forecast using the series as input data. Data from
      one week previous to the series is used to bootstrap the initial forecast.