A Concrete Example
<?xml version="1.0"?>
<!DOCTYPE FAMILYTREE [
<!ELEMENT FAMILYTREE (PERSON | FAMILY)*>
<!-- PERSON elements -->
<!ELEMENT PERSON (NAME*, BORN*, DIED*, SPOUSE*)>
<!ATTLIST PERSON
ID ID #REQUIRED
FATHER CDATA #IMPLIED
MOTHER CDATA #IMPLIED
>
<!ELEMENT NAME (#PCDATA)>
<!ELEMENT BORN (#PCDATA)>
<!ELEMENT DIED (#PCDATA)>
<!ELEMENT SPOUSE EMPTY>
<!ATTLIST SPOUSE IDREF IDREF #REQUIRED>
<!--FAMILY-->
<!ELEMENT FAMILY (HUSBAND?, WIFE?, CHILD*) >
<!ATTLIST FAMILY ID ID #REQUIRED>
<!ELEMENT HUSBAND EMPTY>
<!ATTLIST HUSBAND IDREF IDREF #REQUIRED>
<!ELEMENT WIFE EMPTY>
<!ATTLIST WIFE IDREF IDREF #REQUIRED>
<!ELEMENT CHILD EMPTY>
<!ATTLIST CHILD IDREF IDREF #REQUIRED>
]>
<FAMILYTREE>
<PERSON ID="p1">
<NAME>Domeniquette Celeste Baudean</NAME>
<BORN>21 Apr 1836</BORN>
<DIED>Unknown</DIED>
<SPOUSE IDREF="p2"/>
</PERSON>
<PERSON ID="p2">
<NAME>Jean Francois Bellau</NAME>
<SPOUSE IDREF="p1"/>
</PERSON>
<PERSON ID="p3" FATHER="p2" MOTHER="p1">
<NAME>Elodie Bellau</NAME>
<BORN>11 Feb 1858</BORN>
<DIED>12 Apr 1898</DIED>
<SPOUSE IDREF="p4"/>
</PERSON>
<PERSON ID="p4" FATHER="p2" MOTHER="p1">
<NAME>John P. Muller</NAME>
<SPOUSE IDREF="p3"/>
</PERSON>
<PERSON ID="p7">
<NAME>Adolf Eno</NAME>
<SPOUSE IDREF="p6"/>
</PERSON>
<PERSON ID="p6" FATHER="p2" MOTHER="p1">
<NAME>Maria Bellau</NAME>
<SPOUSE IDREF="p7"/>
</PERSON>
<PERSON ID="p5" FATHER="p2" MOTHER="p1">
<NAME>Eugene Bellau</NAME>
</PERSON>
<PERSON ID="p8" FATHER="p2" MOTHER="p1">
<NAME>Louise Pauline Bellau</NAME>
<BORN>29 Oct 1868</BORN>
<DIED>3 May 1938</DIED>
<SPOUSE IDREF="p9"/>
</PERSON>
<PERSON ID="p9">
<NAME>Charles Walter Harold</NAME>
<BORN>about 1861</BORN>
<DIED>about 1938</DIED>
<SPOUSE IDREF="p8"/>
</PERSON>
<PERSON ID="p10" FATHER="p2" MOTHER="p1">
<NAME>Victor Joseph Bellau</NAME>
<SPOUSE IDREF="p11"/>
</PERSON>
<PERSON ID="p11">
<NAME>Ellen Gilmore</NAME>
<SPOUSE IDREF="p10"/>
</PERSON>
<PERSON ID="p12" FATHER="p2" MOTHER="p1">
<NAME>Honore Bellau</NAME>
</PERSON>
<FAMILY ID="f1">
<HUSBAND IDREF="p2"/>
<WIFE IDREF="p1"/>
<CHILD IDREF="p3"/>
<CHILD IDREF="p5"/>
<CHILD IDREF="p6"/>
<CHILD IDREF="p8"/>
<CHILD IDREF="p10"/>
<CHILD IDREF="p12"/>
</FAMILY>
<FAMILY ID="f2">
<HUSBAND IDREF="p7"/>
<WIFE IDREF="p6"/>
</FAMILY>
</FAMILYTREE>