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Schema languages

A schema defines the syntax of XML documents for a particular application domain.
(- in other words, it defines the grammar for an XML-based markup language.)

DTD (Document Type Definition): XML's native schema language
- not enough expressive power
- horrible syntax

DSD (Document Structure Description): AT&T and BRICS's schema language proposal
- very expressive
- is itself written in XML notation
- based on familiar computer science concepts (regular expressions, boolean logic, ...)
- 100% self-describable
- allows linear-time processing
- is defined in a short, human-readable specification