VMSS, chapter 11 (Repeated inheritance), page 192

     Select Subclause rule

     A Select subclause appearing in the Parent part for a
     class  `B' in a  class  `D'  is valid if and only if,
     every  Feature_identifier `fname' in its Feature_list
     satisfies the following three conditions:

     1 `fname' is the  final name in `D' of a feature that
       has two  or  more  potential  versions  in  `D'.

     2 `fname' appears only once in the Feature_list.

     3 No other Parent part in `D' lists another potential
       version of the same original feature.