document: Expressiveness of XSDs : from practice to theory , there and back again On an abstract level , XML Schema increases the limited expressive power of Document Type Definitions ( DTDs ) by extending them with a recursive typing mechanism . However , an investigation of the XML Schema Definitions ( XSDs ) occurring in practice reveals that the vast majority of them are structurally equivalent to DTDs . This might be due to the complexity of the XML Schema specification and the difficulty to understand the effect of constraints on typing and validation of schemas . To shed some light on the actual expressive power of XSDs this paper studies the impact of the Element Declarations Consistent ( EDC ) and the Unique Particle Attribution ( UPA ) rule . An equivalent formalism based on contextual patterns rather than on recursive types is proposed which might serve as a light-weight front end for XML Schema . Finally , the effect of EDC and UPA on the way XML documents can be typed is discussed . It is argued that a cleaner , more robust , stronger but equally efficient class is obtained by replacing EDC and UPA with the notion of 1-pass preorder typing : schemas that allow to determine the type of an element of a streaming document when its opening tag is met . This notion can be defined in terms of restrained competition regular expressions and there is again an equivalent syntactical formalism based on contextual patterns .
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