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#!/usr/bin/perl -w

use XML::Checker::Parser;
...
    die XML::Checker::error_string ($code, @_) if $code < 200;
    XML::Checker::print_error ($code, @_);
}

<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT PDList (PD*)>
<!ATTLIST PDList RunDateTime CDATA #REQUIRED>
...
<!ELEMENT OrganizationalCode (#PCDATA)>
<!ELEMENT NASAClassCode (#PCDATA)>
<!ELEMENT SourceSystem (#PCDATA)>

<!DOCTYPE PDList SYSTEM "http://ifmp.nasa.gov/PDList.dtd">
<PDList RunDateTime="200203211000">
        <PD PDNumber="10449">
...
                <SourceSystem>Avue version whatever</SourceSystem>
        </PD>
</PDList>