Question: when was the first successful open heart surgery performed
[{'answer': '1956', 'provenance': [{'wikipedia_id': '804779', 'title': 'Cardiac surgery', 'start_paragraph_id': 10, 'start_character': 310, 'end_paragraph_id': 10, 'end_character': 314, 'bleu_score': 1.0, 'section': 'Section::::History.:20th century.\n'}]}, {'provenance': [{'wikipedia_id': '804779', 'title': 'Cardiac surgery', 'start_paragraph_id': 10, 'start_character': 0, 'end_paragraph_id': 10, 'end_character': 401, 'bleu_score': 0.9669306597086381, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8576107545328609692}, 'section': 'Section::::History.:20th century.\n'}]}]