Schema languages

The syntax of a new XML language must be formalized:
*   this is similar to the formal syntax of a programming language;
*   however, usual context-free grammars are not expressive enough;
*   XML languages are described using schemas.


A modern schema language:
*   is itself an XML language (and can be used to describe itself);
*   imposes constraints on the contents of elements;
*   is context-sensitive and very fine-grained;
*   can be processed efficiently.


A schema processor:
*   checks that an application document satisfies the schema;
*   such a document is called valid.