<!ELEMENT bib (book*)>
<!ELEMENT book (title, (author+ | editor+), publisher, price)>
<!ATTLIST book year CDATA #REQUIRED>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (last, first, affiliation?)>
<!ELEMENT last (#PCDATA)>
<!ELEMENT first (#PCDATA)>
<!ELEMENT publisher (#PCDATA)>
<!ELEMENT editor (last, first, affiliation?)>
<!ELEMENT price (#PCDATA)>
<!ELEMENT affiliation (#PCDATA)>