docs: def gaus_pdf(x, mean, std):
      '''Gaussian distribution's probability density function.
  
      See, e.g. `Wikipedia <https://en.wikipedia.org/wiki/Normal_distribution>`_.
  
      :param x: point in x-axis
      :type x: float or numpy.ndarray
      :param float mean: mean or expectation
      :param float str: standard deviation
      :returns: pdf(s) in point **x**
      :rtype: float or numpy.ndarray
      '''
      return exp(-((x - mean) / std)**2 / 2) / sqrt(2 * pi) / std