How to write DTD to constrain multiple IDREF values in a string?

[dieptan]

\[code\]<Course Number="CS105A" Prerequisites="CS101A" Instructors="JC XX" Enrollment="610"> <Title>Programming XXX</Title> <Description>Abstraction and its relation to programming.</Description></Course><Course Number="CS106B" Prerequisites="CS106A" Instructors="JC ER" Enrollment="620"> <Title>Programming Abstractions</Title> <Description>Abstraction and its relation to programming.</Description></Course><Course Number="CS107" Prerequisites="CS106B CS105A" Instructors="JZ" Enrollment="500"> <Title>Computer Organization and Systems</Title> <Description>Introduction to the fundamental concepts of computer systems.</Description></Course>\[/code\]How to write the \[code\]ATTLIST\[/code\] spec for the \[code\]Prerequisites\[/code\] attribute which can be composed of multiple ID values such as \[code\]"CS106B CS105A"\[/code\]? Would the following work?\[code\]<ATTLIST Course Number ID #REQUIRED><ATTLIST Course Prerequisites IDREF #IMPLIED>\[/code\]