  <!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 >
