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Very Simple DTD
<company>
    <person>  <ssn> 123456789 </ssn>
                      <name> John </name>
                      <office> B432 </office>
                      <phone> 1234 </phone>
    </person>
    <person>  <ssn> 987654321 </ssn>
                      <name> Jim </name>
                      <office> B123 </office>
    </person>
    <product> ...  </product>
    ...
</company>
Example of valid XML document: