- IDREF - Hold the value of an ID attribute of some element. Usually another
element to which the current element is related.
<?xml version="1.0" standalone="yes"?>
<!DOCTYPE lab_group [
<!ELEMENT lab_group (student_name)*>
<!ELEMENT student_name (#PCDATA)>
<!ATTLIST student_name student_id ID #REQUIRED
tutor IDREF #IMPLIED>
]>
<lab_group>
<student_name student_id="S8904885">Alex Foo</student_name>
<student_name student_id="S9011133">Sarah Bar</student_name>
<student_name student_id="S9216735"
tutor="S8904885">Jo Smith</student_name>
</lab_group>
