Question: when did snape become defense against the dark arts teacher
[{'answer': 'book six', 'provenance': [{'wikipedia_id': '310355', 'title': 'Magic in Harry Potter', 'start_paragraph_id': 80, 'start_character': 901, 'end_paragraph_id': 80, 'end_character': 909, 'bleu_score': 1.0, 'section': 'Section::::Subjects at Hogwarts.:Defence Against the Dark Arts.\n'}]}, {'provenance': [{'wikipedia_id': '310355', 'title': 'Magic in Harry Potter', 'start_paragraph_id': 80, 'start_character': 0, 'end_paragraph_id': 80, 'end_character': 1797, 'bleu_score': 0.9931152160837011, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 18407864597398758099}, 'section': 'Section::::Subjects at Hogwarts.:Defence Against the Dark Arts.\n'}]}]