VSRT, ECMA Standard 367, section: 8.3.9, page 32

   Root Type Rule

   It is valid to designate a type `TN' as root type of a
   system of universe `U' if and only if it satisfies the
   following conditions:

      1 `TN' is the name of a stand-alone type `T'.

      2 `T' only involves classes in `U'.

      3 `T's base class is not deferred.

      4 The base class of any type on which `T' depends is in `U'.