Question: when did chelsea win the cup winners cup
[{'answer': '13 May 1998', 'provenance': [{'wikipedia_id': '4694400', 'title': "1998 UEFA Cup Winners' Cup Final", 'start_paragraph_id': 1, 'start_character': 77, 'end_paragraph_id': 1, 'end_character': 88, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]