Question: what did the northwest ordinance of 1787 prohibit in the northwest territories
[{'answer': 'slavery', 'provenance': [{'wikipedia_id': '140997', 'title': 'Northwest Ordinance', 'start_paragraph_id': 4, 'start_character': 19, 'end_paragraph_id': 4, 'end_character': 26, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '140997', 'title': 'Northwest Ordinance', 'start_paragraph_id': 25, 'start_character': 0, 'end_paragraph_id': 25, 'end_character': 709, 'bleu_score': 0.9618830282233141, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5885754298724750791}, 'section': 'Section::::Effects of the legislation.:Prohibition of slavery.\n'}]}]