There is no enforcement that an IDREF must point to an ID on a particular element type, simply that is must point to an existing ID, and further that all IDs must be unique.
Having highlighted the fact that the existing schema paradigm can only express constraints among data items in terms of the child and sibling axes, it is natural to consider whether an alternate paradigm might allow a schema author to exploit these additional relationships to define additional types of constraint amongst document elements.
Abstract Schematron [Schematron] is a structural based validation language, defined by Rick Jelliffe, as an alternative to existing grammar based approaches.
Tree patterns, defined as XPath expressions, are used to make assertions, and provide user-centred reports about XML documents.
The Schematron conformance language for custom implementation is also introduced.
The paper completes with some suggestions of possible future extensions.
Schematron is a useful and accessible supplement to other schema languages.
This innovation stems from selecting an alternative approach to validation than existing schema languages: Schematron uses a tree pattern based paradigm, rather than the regular grammars used in DTDs and XML schemas.This comes at very little cost: XPath is available in most XML environments.For example the following types of constraint are hard, or impossible to express with other schema languages.Other tutorial materials fulfill these roles already [Holman],[XPath],[XSLT] ,[Ogbuji C],[Ogbuji U].During the last few years a number of different XML schema languages have appeared as suggested replacements for the ageing Document Type Definition (DTD).
Search for validating statements:
Expressing validation rules using patterns is often easier than defining the same rule using a content model.