<!ELEMENT album (track*)+>
<!ATTLIST album
title CDATA #IMPLIED
 aid CDATA #IMPLIED
>
<!ELEMENT catalogue (group)>
<!ELEMENT group (album*)>
<!ATTLIST group
 name CDATA #IMPLIED
>
<!ELEMENT track (#PCDATA)>
<!ATTLIST track
 tid CDATA #IMPLIED
 id CDATA #IMPLIED
>

