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