Sequential pattern mining is to mine sequential patterns for a set of items occurring in sequence order. In practice, people may like to find sequential patterns for types of items instead of for concrete items, such as sequence patterns formed by high-level concepts. For example, instead of finding sequential patterns composed of concrete models of i-phones in shopping transactions, but finding patterns composed of Apple products, smart-phones, electronics, and so on. Outline an efficient sequential pattern mining algorithm that simultaneously mines sequential patterns at multiple levels of abstraction.