VDRS, chapter 10 (Feature adaptation), page 159

     Redefine Subclause rule

     Consider a class `C' with a parent `B'. If  a  Parent
     part  for  `B'  in `C' contains a Redefine subclause,
     that  clause  is  valid  if   and   only   if   every
     Feature_identifier  `fname'  that  it  lists  (in its
     Feature_list) satisfies the following conditions:

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

        2 That feature was  not  frozen,  and  was  not  a
          constant attribute.

        3 `fname' appears only once in the Feature_list.

        4  The  Features  part   of   `C'   contains   one
          Feature_declaration  for  `fname',  which  is  a
          valid redeclaration, but not  an  effecting,  of
          the original feature.