@abc.abstractmethod<EOL><INDENT>def dlogprior(self, param):<DEDENT>
pass<EOL>
Derivative of `logprior` with respect to `param`.

        Args:
            `param` (string in `freeparams`)

        Returns:
            Derivative of `logprior` with respect to `param`,
            or 0 if there is no prior defined over this model.