VDUS, chapter 10 (Feature adaptation), page 160

     Undefine Subclause rule

     Consider a class `C' that inherits from a class  `B'.
     If  a Parent part for `B' in `C' contains an Undefine
     subclause, that clause is valid if and only  if,  for
     every  Feature_identifier  `fname'  that it lists (in
     its Feature_list):

        1 `fname' is the final name in `C'  of  a  feature
          inherited from `B'.

        2 That feature was not  frozen,  and  was  not  an
          attribute.

        3 That feature was effective in `B'.

        4 `fname' appears only once in the Feature_list.