<?xml  version="1.0"?>
<!-- validity constraint check for Production 56(Positive Test)-->
<!DOCTYPE test
 [
 <!ELEMENT test ANY>
 <!ELEMENT id1 EMPTY>
 <!ELEMENT id2 EMPTY>
 <!ELEMENT idref EMPTY>
 <!ATTLIST id1 UniqueName ID #REQUIRED>
 <!ATTLIST id2 UName ID #IMPLIED>
 <!ATTLIST idref reference IDREFS #IMPLIED>
 ]>
<test>
<id1 UniqueName = "AC456"></id1>
<id2 UName = "Q123"></id2>
<idref reference = "AC456 Q123"></idref>
Positive test for validity constraint of IDREFS.
In an attribute decl, values of type IDREFS match tha name production
and the IDREFS value matches the values assigned to an ID attributes somewhere
in the XML document.
</test>