VTEC, chapter 12 (Types), page 209

     Expanded Type rule

     It is valid to use an expanded type of base class `C'
     in  the  text  of  a  class  `B'  if  and  only if it
     satisfies the following two conditions:

        1 `C' is not a deferred class.

        2 `C' either  has  no  Creators  part,  or  has  a
          Creators  part  containing  exactly one creation
          procedure, with no argument,  available  to  `B'
          for creation.