Finished DTD

<!ELEMENT category     (#PCDATA)>
<!ELEMENT abstract     (#PCDATA)>
<!ELEMENT keyword      (#PCDATA)>
<!ELEMENT last_updated (#PCDATA)>
<!ELEMENT copyright    (#PCDATA)>
<!ELEMENT title        (#PCDATA)>
<!ELEMENT instruments  (#PCDATA)>
<!ELEMENT publisher    (#PCDATA)>
<!ELEMENT length       (#PCDATA)>

<!ELEMENT date    (year | ISODate)>
<!ELEMENT year    (#PCDATA)>
<!ELEMENT ISODate (#PCDATA)>

<!ELEMENT catalog (category, cataloging_info, last_updated, 
   copyright, maintainer, (composer | composition)*)>

<!ELEMENT cataloging_info (abstract, keyword+)>

<!ELEMENT description (#PCDATA | ul | a | cite | p)*>
<!ELEMENT cite        (#PCDATA | a)*>
<!ELEMENT ul          (li*)>
<!ELEMENT li          (#PCDATA)>
<!ELEMENT a           (#PCDATA)>
<!ELEMENT p           (#PCDATA)>

<!ELEMENT maintainer  (name)>
<!ELEMENT name        (first_name, middle_name, last_name)>
<!ELEMENT first_name  (#PCDATA)>
<!ELEMENT middle_name (#PCDATA)>
<!ELEMENT last_name   (#PCDATA)>
<!ATTLIST maintainer email CDATA #REQUIRED 
                     url   CDATA #IMPLIED>
                     
<!ELEMENT composer (name)>
<!ATTLIST composer id ID #REQUIRED>

<!ELEMENT composition (title, date, length?, instruments, description?, publisher?)>
<!ATTLIST composition composer IDREFS #REQUIRED>

<!ATTLIST a href CDATA #REQUIRED>