IDREF represents the IDREF attribute type from [XML 1.0 Recommendation]. The value space of IDREF is the set of all strings
that match the NCName production in [Namespaces in XML] and have been used in an XML document as the value of an element or
attribute of type ID. The lexical space of IDREF is the set of strings that match the NCName production in [Namespaces in
XML].