Question: who is the leader of the senate when the vp isn't there
[{'answer': 'President pro tempore', 'provenance': [{'wikipedia_id': '32759', 'title': 'Vice President of the United States', 'start_paragraph_id': 16, 'start_character': 406, 'end_paragraph_id': 16, 'end_character': 427, 'bleu_score': 1.0, 'section': 'Section::::Roles of the vice president.:Preside over the United States Senate.\n'}]}]