Extending XML with nonmonotonic multiple inheritance

Schema descriptions of XML documents become more and more complicated and schema documents become longer and longer as the structure of XML documents becomes more and more complex. This is mainly because they cannot take full use of object-oriented modeling abilities. In this paper, we extend XML as follows to solve this problem. (1) We extend DTD's type system to provide richer built-in types. Moreover, a user-defined type can be declared using the ISA mechanism in which an existing type is used as the base type and the set of values represented by the derived type is the subset of values represented by the base type. (2) We extend DTD so that element can be global as well as local. (3) We extend DTD with element hierarchy with nonmonotonic inheritance to support super-element sub-element relationship, overriding of elements or attributes inherited from super-elements, blocking of the inheritance of elements or attributes from super-elements, and conflict handling. (4) We extend XML with polymorphism, which is a fundamental feature in object-oriented data models, to support polymorphic elements, typing of references and polymorphic references. Although we extend DTD to support some key object-oriented features, there is not any syntax change of XML documents to fit for our Extended DTD.