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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def stddev(values, meanval=None):
 #from AI: A Modern Appproach """The standard deviation of a set of values. Pass in the mean if you already know it."""

    if meanval == None: meanval = mean(values)
    return math.sqrt( sum([(x - meanval)**2 for x in values]) / (len(values)-1) )