DTD for attributes revised.

DTD for attributes revised.
New syntax:
 <attributes>
  <class type="..">
   ...
  </class>
  <generated />
 </attributes>
where the ... are the arguments of the class and the class and generated
elements are optional. For the "elim" class the only argument is the SORT.
For the "record" class the arguments are a list of <field name=".."/>