InputClassroom:
	'Classroom ' classroomName=[ClassroomDefinition] ' has students : '
	(students+=InputStudent)*
InputStudent:
	chosenStudent=[StudentDefinition] state

enum state:
	' present ' | ' missing '
	
ClassroomDefinition:
	'Classroom ' classRoomName=ID
	(classroomStudents+= StudentDefinition)*

StudentDefinition:
	'Student ' studentName=ID

And the database looks like:

Classroom B442
	Annie
	Bob
	Zoe

Student Annie
	....
Student Bob
	...
Student Zoe
	...

public IScope scope_ InputStudent _ chosenStudent (InputStudent inputStudent, EReference ref) {
	StudentDefinition nextStudent = ((InputClassroom)inputStudent.eContainer()).getClassroomDefinition(). GetClassroomStudent ().size()-1);
	@SuppressWarnings("unchecked")
EList<EObject> choices = (EList<EObject>) Proxy.newProxyInstance(Student.class.getClassLoader(),
				new Class [] {EList.class}, new InvocationHandler() {
					private List< StudentDefinition> list = new ArrayList< StudentDefinition> ();
					
					@Override
					public Object invoke(Object obj, Method meth, Object[] args)
							throws Throwable {
						return meth.invoke(list, args);
					}
				});
		choices.add(nextStudent);
		return Scopes.scopeFor(choices);
	}