<!--DOCTYPE book [-->
  <!ELEMENT book (title, author+, section+)>
  <!ELEMENT title (#PCDATA)>
  <!ELEMENT author (#PCDATA)>
  <!ELEMENT section (title, (p | figure | section)* )>
  <!ATTLIST section
      id ID #IMPLIED
      difficulty CDATA #IMPLIED>
  <!ELEMENT p (#PCDATA)>
  <!ELEMENT figure (title, image)>
  <!ATTLIST figure
       width CDATA #REQUIRED
       height CDATA #REQUIRED >
  <!ELEMENT image EMPTY>
  <!ATTLIST image
       source CDATA #REQUIRED>
<!--]-->