<?xml  version="1.0"?>
<!-- validity constraint check for Production 56(positive Test)-->
<!DOCTYPE tokenizer
 [
 <!ELEMENT tokenizer ANY>
 <!ELEMENT b EMPTY>
 <!ATTLIST b attr ID #REQUIRED>
 <!ATTLIST tokenizer UniqueName ID #REQUIRED>
  ]>
<tokenizer UniqueName = "Ac999">
<b attr = "BC999"></b>
This is a positive test for validity constraints
the value of the attribute with a type ID does not appear more than once in the XML document
</tokenizer>