This module provides functions for checking special IDIDREFIDREFS constraints.

Checking special ID/IDREF/IDREFS constraints means:

• checking that all ID values are unique.
• checking that all IDREF/IDREFS values match the value of some ID attribute

ID-Validation should be started before or after validating the document.

First all nodes with ID attributes are collected from the document, then it is validated that values of ID attributes do not occure more than once. During a second iteration over the document it is validated that there exists an ID attribute value for IDREF/IDREFS attribute values.