look for the tags they need, and ignore all the others. At a more detailed level, an agreement should also constrain the structure. For example a data item with a price tag must contain an integer value, while a data item with a product tag must contain nested items labeled name, price, and description. An agreement on the structure enables applications to navigate the data in a meaningful way. We call a collection of constraints on the structure a type. Several type formalism have been proposed for semistructured data [BDFS97, GW97, BM99], and several are considered for XML [Con98, BLM + 99, BFRW84]. There is an obvious analogy between types in semistructured data types in programming languages. But there is an important dierence. The former are global constraints on the data, while the latter are local constraints. For example, if a