ID attributes and points-to requirements

- simple definitions and references inside a document.

A DSD can declare special application document attribute types:

An ID attribute is a definition of its value (must be unique in that document).
An IDRef attribute is a reference to the element containing the definition of the value.

(ID and IDRef originate from DTD.)

A DSD may also impose a points-to requirement on a reference: this is a boolean expression which must evaluate to true on the referenced node. This allows semi-structured data to be expressed.

Special attribute types are declared along with the attribute. Example:

<AttributeDecl Name="book-reference" IDType="IDRef"> <PointsTo> <Context><Element Name="book"></Context> </PointsTo> </AttributeDecl>

(a book-reference attribute has type IDRef and must refer to an attribute of type ID occuring in a book element.)