VDJR, section 8.10.28

     Join Validity rule

     It is valid for a class C to inherit two different
     features under the same final name under and only
     under the following conditions:

        1 After possible redeclaration in C, their
          signatures are identical.

        2 They either have both no aliases or have the
          same alias.

        3 If they both have assigner commands, the
          associated procedures have the same final
          name in C.

        4 If both are inherited as effective, C redefines
          both into a common version.