VLEL, chapter 7 (Clients and exports), page 102

     Export List rule

     A New_exports parent appearing  in  class  `C'  in  a
     Parent clause for a parent `B', of the form

     export

        {`class_list1'} `feature_list1';
        ...
        {`class_listn'} `feature_listn';

     is valid if and only if  (for  `i'  in  the  interval
     1..`n'):

        1 At  most  one  of  the  `feature_listi'  is  the
          keyword all.

        2 All the other `feature_listi' are lists of final
          names of features of `C' obtained from `B'.

        3 No final feature name appears twice in any  such
          list, or appears in more than one list.
