def _logpdf(self, **kwargs):
        """Returns the log of the pdf at the given values. The keyword
        arguments must contain all of parameters in self's params. Unrecognized
        arguments are ignored.
        """
        if kwargs in self:
            return sum([self._lognorm[p] +
                        self._expnorm[p]*(kwargs[p]-self._mean[p])**2.
                        for p in self._params])
        else:
            return -numpy.inf