<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def holtWintersForecast(requestContext, seriesList):
 """ 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. """

    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