ex2-1.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE BookCatalog
[<!ELEMENT BookCatalog  (Book)*>
<!ELEMENT Book          (Title, Author, Year)>
<!ELEMENT Title         (#PCDATA)>
<!ELEMENT Author        (#PCDATA)>
<!ELEMENT Year          (#PCDATA)>
]>
<BookCatalog>
    <Book>
        <Title>new ideas from dead economists</Title>
        <Author>Todd G. Buchholz</Author>
        <Year>2007</Year>
    </Book>
    <Book>
        <Title>Pensées</Title>
        <Author>Blaise Pascal</Author>
        <Year>1670</Year>
    </Book>
    <Book>
        <Title>Zero to One</Title>
        <Author>Peter Thiel</Author>
        <Year>2014</Year>
    </Book>
    <Book>
        <Title>Gang Leader for a Day</Title>
        <Author>Sudhir Venkatesh</Author>
        <Year>2008</Year>
    </Book>
</BookCatalog>