docs: def _ss(data, c=None):
      """Return sum of square deviations of sequence data.
  
      If ``c`` is None, the mean is calculated in one pass, and the deviations
      from the mean are calculated in a second pass. Otherwise, deviations are
      calculated from ``c`` as given. Use the second case with care, as it can
      lead to garbage results.
      """
      if c is None:
          c = mean(data)
      #print(data)
      ss = _sum((x-c)**2 for x in data)
      # The following sum should mathematically equal zero, but due to rounding
      # error may not.
      ss -= _sum((x-c) for x in data)**2/len(data)
      assert not ss < 0, 'negative sum of square deviations: %f' % ss
      return ss