<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE marvel [
  <!NOTATION JPG SYSTEM "image/jpeg">
  <!NOTATION GIF SYSTEM "image/gif">
  <!NOTATION PNG SYSTEM "image/png">
  
  <!ELEMENT marvel (superheroe*)>
  <!ELEMENT superheroe (#PCDATA)>
  <!ATTLIST superheroe nombre CDATA #REQUIRED
                       tipoImagen NOTATION (GIF | JPG | PNG) #IMPLIED>
]>
<marvel>
  <superheroe nombre="Thor" tipoImagen="PNG">Un Dios</superheroe>
</marvel>