Question: when was slavery defined in law and custom
[{'answer': '1807', 'provenance': [{'wikipedia_id': '39104703', 'title': 'Slavery in international law', 'start_paragraph_id': 5, 'start_character': 34, 'end_paragraph_id': 5, 'end_character': 38, 'bleu_score': 1.0, 'section': 'Section::::Historical abolition of slavery.\n'}]}, {'provenance': [{'wikipedia_id': '39104703', 'title': 'Slavery in international law', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 424, 'bleu_score': 0.9432477065696491, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9002054829473454648}, 'section': 'Section::::Historical abolition of slavery.\n'}]}]