例子:
>>> process_purchase([('rice', 'mitsuwa'), ('msg', '99ranch'), \
('eggs', 'costco')])
得出{'mitsuwa': ['rice'], '99ranch': ['msg'], 'costco': ['eggs']}
>>> process_purchase([('milk', 'ralphs'), ('carrot', 'ralphs'), \
('milk', 'ralphs'), ('carrot', 'costco')])
得出{'ralphs': ['milk', 'carrot', 'milk'], 'costco': ['carrot']}