<?xml  version="1.0"?>
<!-- validity constraint check for Production 56(Negative 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 = "BC456"></idref>
Negative test for validity constraint of IDREF.
In an attribute decl, values of type IDREF match the name production and
IDREF value does not match the value assigned to any ID attribute somewhere
in the XML document.
</test>