document: Schemapath , a minimal extension to xml schema for conditional constraints In the past few years , a number of constraint languages for XML documents has been proposed . They are cumulatively called schema languages or validation languages and they comprise , among others , DTD , XML Schema , RELAX NG , Schematron , DSD , xlinkit . One major point of discrimination among schema languages is the support of co-constraints , or co-occurrence constraints , e.g. , requiring that attribute A is present if and only if attribute B is ( or is not ) presentin the same element . Although there is no way in XML Schema to express these requirements , they are in fact frequently used in many XML document types , usually only expressed in plain human-readable text , and validated by means of special code modules by the relevant applications . In this paper we propose SchemaPath , a light extension of XML Schema to handle conditional constraints on XML documents . Two new constructs have been added to XML Schema : conditions -- based on XPath patterns -- on type assignments for elements and attributes ; and a new simple type , xsd : error , for the direct expression of negative constraints ( e.g. it is prohibited for attribute A to be present if attribute B is also present ) . A proof-of-concept implementation is provided . A Web interface is publicly accessible for experiments and assessments of the real expressiveness of the proposed extension .
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