Question: when did federer last won a grand slam
[{'answer': '2008', 'provenance': [{'wikipedia_id': '262376', 'title': 'Roger Federer', 'start_paragraph_id': 3, 'start_character': 517, 'end_paragraph_id': 3, 'end_character': 521, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]