Question: who was president of mexico during the mexican american war
[{'answer': 'Antonio López de Santa Anna', 'provenance': [{'wikipedia_id': '21073732', 'title': 'Mexican–American War', 'start_paragraph_id': 1, 'start_character': 463, 'end_paragraph_id': 1, 'end_character': 490, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]