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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_default_reference(self, method):
 """ Returns the default reference for a method. :arg method: name of a method :type method: :class:`str` :return: reference :rtype: :class:`Reference <pyxray.descriptor.Reference>` or :class:`str` """

        if method not in self._available_methods:
            raise ValueError('Unknown method: {0}'.format(method))
        return self._default_references.get(method)