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 or once class.

        2 `C''s version of  the procedure `default_create'
          (inherited from `ANY') is one  of  the  creation
          procedures of `C' available to `B' for creation.