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For example, we may select all Regular grammars, as used in DTDs, can then be viewed as tree patterns where the only available axis is the parent-child axis [Jelliffe1999e].Full use of tree pattern validation provides the maximum amount of freedom when modelling constraints for a schema.Tree patterns do just that, and XPath provides a convenient syntax in which to express those patterns.Validation using tree patterns is a two-step process: Both the candidate object selection, and the assertions can be defined in terms of XPath expressions.Tree patterns are the schema paradigm underpinning Schematron as a validation language.There are reasons to believe that tree-pattern validation may be more suitable in an environment where documents are constructed from elements in several namespaces (often termed 'data islands').
The later sections in this paper provides an overview of the open source XSLT framework used to implement the Schematron language.
As the XPath specification [XPath] shows, there are many possible kinds of relationship, known as 'axes'.
While XML does include an ID/IDREF mechanism which allows for cross-referencing between elements, and hence another form of relationships, it only weakly binds those elements.
The majority of these have taken the basic premise of recasting DTD functionality in XML syntax with the addition, in some cases, of other features such as data typing, inheritance, etc [XMLSchema].
The use of XML syntax provides additional flexibility through leveraging existing tools for markup manipulation, while the 'value added' features satisfy the requirements of developers looking for closer integration with databases and object-oriented languages.