Class mobility

Each model class is defined as a parameter entity, containing a reference to an initially null extension class, and a list of members
```
 <!ENTITY % x.class "" >
<!ENTITY % m.class "%x.class; name1|name2|name3 ..." >
<!ELEMENT % n.element - - (%m.class;+)>  
```
To add a new member to a class, we redefine the extension class:
```
<!ENTITY % x.class "myChunk|myOther|"> 
```

TEI and XML: a marriage made in heaven?