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In pervasive computing applications, how to effectively manage and query XML data streams is an important issue. Particularly, query rewriting techniques for processing complex query which can cause ambiguities and redundant query cost, have been proposed to simplify query and remove its redundancy. However, less attention has been paid to make use of query rewriting to benefit the query evaluation process of an efficient XML data stream query system. In this paper, we propose a normalization process which rewrites the Forward XPath expressions for our XML data stream query system. The proposed normalization is expected to normalize all the complex predicates connected with logic operators into twig structures. As a result, the formal way of the normalization makes the query language of our XML data stream query system more precise and the generated twig structure facilitates the query evaluation of our query system.