Crear un DTD
<?xml version="1.0" standalone="no"?>
<!DOCTYPE NOVEL [
<!ELEMENT NOVEL (FORWORD,CHAPTER+,BIOGRAPHY?,CRITICALESSAY*)>
<!ELEMENT FORWORD (PARAGRAPH+)>
<!ELEMENT CHAPTER (SECTION+|PARAGRAPH+)>
<!ELEMENT SECTION (PARAGRAPH+)>
<!ELEMENT BIOGRAPHY(PARAGRAPH+)>
<!ELEMENT CRITICALESSAY (SECTION+)>
<!ELEMENT PARAGRAPH (#PCDATA)>
<!ELEMENT SIGN (#PCDATA)>
]>