<?xml  version="1.0"?>
<!-- validity constraint check for Production 56(positive Test)-->
<!DOCTYPE tokenizer
 [
 <!ELEMENT tokenizer ANY>
 <!ELEMENT a EMPTY>
 <!ELEMENT b EMPTY>
 <!ATTLIST a first ID #REQUIRED>
 <!ATTLIST b second ID #REQUIRED>
 ]>
<tokenizer>
<a first = "AC1999"></a>
<b second = "CD345"></b>
This is a positive validity test for ID.
any element type has no more than one attribute of type ID specified
</tokenizer>