<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

<!-- root ELEMENT   student   contains a choice of only one   name | student-id | ssn   elements -->
<!-- use | bars between the elements when you specify a choice -->

<!DOCTYPE student 
  [
  <!ELEMENT student (name | student-id | ssn) >
  <!ELEMENT name EMPTY >
  <!ELEMENT student-id EMPTY >
  <!ELEMENT ssn EMPTY >
 ] >

<student>
  <ssn></ssn>  
</student>