VHPR, section 8.6.13

     Parent rule

     The unfolded inheritance part of a class D is
     valid if and only if it satisfies the following
     conditions:

        1 In every Parent part for a class B, B is
          not a descendant of D.

        2 No conforming parent is a frozen class.

        3 If two or more Parent parts are for classes
          which have a common ancestor A, D meets the
          conditions of the Repeated Inheritance
          Consistency constraint for A.

        4 At least one of the Parent parts is
          conforming.

        5 No two ancestor types of D are different
          generic derivations of the same class.

        6 Every Parent is generic-creation-ready.