Question: when did derek shepherd leave grey's anatomy
[{'answer': '2015', 'provenance': [{'wikipedia_id': '7619908', 'title': 'Derek Shepherd', 'start_paragraph_id': 2, 'start_character': 103, 'end_paragraph_id': 2, 'end_character': 107, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '7619908', 'title': 'Derek Shepherd', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 639, 'bleu_score': 0.540368604589573, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 6441121698084684925}, 'section': 'Section::::Abstract.'}]}]