Question: when did brad stevens become the celtics coach
[{'answer': '2013', 'provenance': [{'wikipedia_id': '10973779', 'title': 'Brad Stevens', 'start_paragraph_id': 4, 'start_character': 65, 'end_paragraph_id': 4, 'end_character': 69, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '10973779', 'title': 'Brad Stevens', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 511, 'bleu_score': 0.77939851015784, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10221109303651889834}, 'section': 'Section::::Abstract.'}]}]