Question: when did helmets become mandatory in the nhl
[{'answer': 'August 1979', 'provenance': [{'wikipedia_id': '568175', 'title': 'Hockey helmet', 'start_paragraph_id': 17, 'start_character': 3, 'end_paragraph_id': 17, 'end_character': 14, 'bleu_score': 1.0, 'section': 'Section::::Helmets in the National Hockey League.\n'}]}, {'provenance': [{'wikipedia_id': '568175', 'title': 'Hockey helmet', 'start_paragraph_id': 17, 'start_character': 3, 'end_paragraph_id': 17, 'end_character': 587, 'bleu_score': 0.8090910319672571, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10905464544510004476}, 'section': 'Section::::Helmets in the National Hockey League.\n'}]}]