Question: leonardo is know for his techniques of shading called
[{'answer': 'sfumato', 'provenance': [{'wikipedia_id': '165442', 'title': 'Sfumato', 'start_paragraph_id': 1, 'start_character': 227, 'end_paragraph_id': 1, 'end_character': 234, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '165442', 'title': 'Sfumato', 'start_paragraph_id': 1, 'start_character': 346, 'end_paragraph_id': 1, 'end_character': 543, 'bleu_score': 0.588228130442984, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2874229733798713484}, 'section': 'Section::::Abstract.'}]}]