@classmethod
def pdf(this, x, *params, **kwargs):
    """Probability density function at x.

        Parameters
        ----------
        x : array_like
           points where to evaluate the pdf
        param1, param2, ... : array_like
           parameters of the model

        Returns
        -------
        pdf : ndarray
           Probability density function evaluated at x

        """
    raise NotImplementedError
