1 <!DOCTYPE doc

     2 [

     3 <!ELEMENT doc (a|refs)*>

     4 <!ELEMENT a EMPTY>

     5 <!ELEMENT refs EMPTY>

     6 <!ATTLIST refs refs IDREFS #REQUIRED>

     7 <!ATTLIST a id ID #REQUIRED>

     8 ]>

     9 <doc>

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    11 <refs refs="A1 A2 A3"/>

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    15 </doc>