docs: def logistic(x, x0, k, L):
      '''Logistic function.
  
      See, e.g `Wikipedia <https://en.wikipedia.org/wiki/Logistic_function>`_.
  
      :param x: point in x-axis
      :type x: float or numpy.ndarray
      :param float x0: sigmoid's midpoint
      :param float k: steepness of the curve
      :param float L: maximum value of the curve
      :returns: function's value(s) in point **x**
      :rtype: float or numpy.ndarray
      '''
      return L / (1 + exp(-k * (x - x0)))