Question: who wrote the words to wagner's opera tristan und isolde
[{'answer': 'Gottfried von Strassburg', 'provenance': [{'wikipedia_id': '38241', 'title': 'Tristan und Isolde', 'start_paragraph_id': 1, 'start_character': 255, 'end_paragraph_id': 1, 'end_character': 279, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '38241', 'title': 'Tristan und Isolde', 'start_paragraph_id': 1, 'start_character': 8, 'end_paragraph_id': 1, 'end_character': 649, 'bleu_score': 0.9210348541089921, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5594222087290700319}, 'section': 'Section::::Abstract.'}]}]