return new Ref(mg.getLocator(),def);
}
public Leaf modelGroup(XSModelGroup mg) {
XSParticle[] children = mg.getChildren();
if(children.length==0) return new Empty(mg.getLocator());
Leaf l = particle(children[0]);
for( int i=1; i<children.length; i++ )
l.merge(particle(children[i]));
return l;