Question: when did the mint start making proof sets
[{'answer': '1936', 'provenance': [{'wikipedia_id': '33396328', 'title': 'United States Mint coin sets', 'start_paragraph_id': 1, 'start_character': 81, 'end_paragraph_id': 1, 'end_character': 85, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '33396328', 'title': 'United States Mint coin sets', 'start_paragraph_id': 18, 'start_character': 0, 'end_paragraph_id': 18, 'end_character': 1146, 'bleu_score': 0.977619558732261, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11475960499685643780}, 'section': 'Section::::United States Mint Proof Set.\n'}]}]