<?xml  version="1.0"?>
<!-- validity constraint check for Production 56(Positive Test)-->
<!DOCTYPE test
 [
 <!ELEMENT test ANY>
 <!ELEMENT id EMPTY>
 <!ELEMENT idref EMPTY>
 <!ATTLIST id UniqueName ID #REQUIRED>
 <!ATTLIST idref reference IDREF #IMPLIED>
 ]>
<test>
<id UniqueName = "AC456"></id>
<idref reference = "AC456"></idref>
Positive test for validity constraint of IDREF.
In an attribute decl, values of type IDREF match tha name production
and the IDREF value matches the value assigned to an ID attribute somewhere
in the XML document.
</test>