#internal.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE bookstore [
    <!ELEMENT bookstore (book+)>
    <!ELEMENT book (title, author, year, price)>
    <!ELEMENT title (#PCDATA)>
    <!ELEMENT author (#PCDATA)>
    <!ELEMENT year (#PCDATA)>
    <!ELEMENT price (#PCDATA)>
    <!ATTLIST book category CDATA #REQUIRED>
]>
<bookstore>
    <book category="fiction">
        <title>Harry Potter</title>
        <author>J.K. Rowling</author>
        <year>1997</year>
        <price>29.99</price>
    </book>
    <book category="non-fiction">
        <title>Sapiens</title>
        <author>Yuval Noah Harari</author>
        <year>2014</year>
        <price>19.99</price>
    </book>
</bookstore>
#external.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE bookstore SYSTEM "bookstore.dtd">
<bookstore>
    <book category="fiction">
        <title>Harry Potter</title>
        <author>J.K. Rowling</author>
        <year>1997</year>
        <price>29.99</price>
    </book>
    <book category="non-fiction">
        <title>Sapiens</title>
        <author>Yuval Noah Harari</author>
        <year>2014</year>
        <price>19.99</price>
    </book>
</bookstore>
#exrternal.dtd
<!ELEMENT bookstore (book+)>
<!ELEMENT book (title, author, year, price)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT price (#PCDATA)>
<!ATTLIST book category CDATA #REQUIRED>