def idxterms(self):
        """List of index terms."""
        try:
            terms = listify(self._json.get("idxterms", {}).get('mainterm', []))
        except AttributeError:  # idxterms is empty
            return None
        try:
            return [d['$'] for d in terms]
        except AttributeError:
            return None
List of index terms.