Example:

<people>
<student status='freshman'><name>Jimmy Dean</name></student>
<professor><name>Bob Glushko</name></professor>
<lacky><name>Alex Milowski</name></lacky>
</people>

Each element 'student', 'professor', and 'lacky' could have a type derived from type 'person'.

The model for 'people' says things of type 'person' goes here.