Example 1: a Thematic Directory

Topics are classified into categories and sub-categories

References to indexed documents appears at the leaves of the hierarchie

Transport: plane rail boat

<!ELEMENT doc (title url keywords)>
...

Semantic constraint: plane, rail, etc. must have a sense in the context of transport

depends on the context
refers to some external knowledge (thesaurus)

A sufficient number of keywords used in the description of the indexed documents must also be relevant in the categorie in which this document is classified

As well as the syntax (the structure) of the document, one want to declare semantic constraints for it