  <!ELEMENT book (quantity,(paperback|hardback),shelving)>
  <!ELEMENT quantity (#PCDATA)>
  <!ELEMENT shelving (shelve+)>
  <!ELEMENT shelve (#PCDATA)>
  <!ELEMENT paperback EMPTY>
  <!ELEMENT hardback EMPTY>
  <!ATTLIST book title CDATA #REQUIRED >
  <!ATTLIST book author CDATA #REQUIRED >
