DTD++ 2.0: Adding support for co- constraints Davide Fiorello

In this paper we present an evolution of the DTD++ schema language for XML documents. The original DTD++ language provided support for a large and significant subset of XML Schema while maintaining a syntax closely resembling DTDs: thus the expressive power of XML Schema and the readability of DTDs were both supported in a modular architecture that could rely on a number of validating engine for XML schema. DTD++ did not provide solution for another big disadvantage of XML Schema and all other grammar-based validation languages: the inability (or limited ability) to express requirements on the co-existence or co-absence of elements and attributes, and other kind of reciprocal constraints cumulatively called co-constraints. The new version of DTD++, uninspiredly called DTD++ 2.0 and presented in this paper, provides a solution for specifying and verifying co-constraints on XML documents: by relying on one additional construct (conditional typ