//Internal DTD
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE catalog [
    <!ELEMENT catalog (book+)>
    <!ELEMENT book (title, author, year, price)>
    <!ELEMENT title (#PCDATA)>
    <!ELEMENT author (#PCDATA)>
    <!ELEMENT year (#PCDATA)>
    <!ELEMENT price (#PCDATA)>
]>
<catalog>
    <book>
        <title>Learning XML</title>
        <author>John Doe</author>
        <year>2024</year>
        <price>29.99</price>
    </book>
    <book>
        <title>Mastering XML</title>
        <author>Jane Smith</author>
        <year>2023</year>
        <price>39.99</price>
    </book>
</catalog>