VWST, chapter 23 (Expressions and constants), page 397

     A Strip expression appearing in a class `C' is  valid
     if  and  only  if  its  Attribute_list  satisfies the
     following two conditions:

        1 Every Identifier appearing in the  list  is  the
          final name of an attribute of `C'.

        2 No Identifier appears twice in the list.