<?xml version="1.0"?>
<!DOCTYPE DOCUMENT [
   <!ELEMENT DOCUMENT (PERSON*)>
   <!ELEMENT PERSON   (#PCDATA)>
   <!ATTLIST PERSON PNUMBER ID #REQUIRED>
   <!ATTLIST PERSON FATHER IDREF #IMPLIED>
   <!ATTLIST PERSON MOTHER IDREF #IMPLIED>
]>
<DOCUMENT>
  <PERSON PNUMBER="a1">Susan</PERSON>
  <PERSON PNUMBER="a2">Jack</PERSON>
  <PERSON PNUMBER="a3" MOTHER="a1" FATHER="a2">Chelsea</PERSON>
  <PERSON PNUMBER="a4" MOTHER="a1" FATHER="a2">David</PERSON>
</DOCUMENT>
