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XML is the de facto standard for storing and exchanging data between different applications. More and more data is being published in the XML format and thus available for further querying and management. Recently, World Wide Web Consortium proposed a new standard XML query language called XQuery. Several procedure-based query interpreter for XQuery have been proposed, most notably Kweelt. We are developing Rainbow, an XML data management system, that at its core has an algebra based XQuery query engine. The XML algebra, which we have developed in the Rainbow project is called XML Algebra Tree (XAT). This XAT has been designed with two primary goals namely querying both relational data and also querying native XML data. This algebra represents a solid foundation for query optimization, computation pushdown, query rewrite, different operator implementations, and other algebra related research tasks in Rainbow as well as in general.