





	Una primera versin de poem.dtd:

         <!ELEMENT anthology      - -  (poem+)>
         <!ELEMENT poem           - -  (title?, stanza+)>
         <!ELEMENT title          - O  (#PCDATA) >
         <!ELEMENT stanza         - O  (line+)   >
         <!ELEMENT line           O O  (#PCDATA) >


	Una segunda versin de poem.dtd:
 

     <!ELEMENT refrain - - (#PCDATA | line+)>
     <!ELEMENT poem    - O (title?,
                           ( (line+)
                           | (refrain?, (stanza, refrain?)+ ) )) >
 

	Una tercera versin de poem.dtd:

         <!DOCTYPE anthology [
         <!ELEMENT anthology      - -  (poem+)             >
         <!ELEMENT poem           - -  (title?, stanza+)   >
         <!ELEMENT stanza         - O  (line+)             >
         <!ELEMENT (title | line) - O  (#PCDATA)           >
         ]>
 
