VHPR, chapter 6 (The inheritance relation), page 79

     Parent rule

     The Inheritance clause of a class `D' is valid if and
     only if it meets the following two conditions:

        1 In every Parent clause for a class `B',  `B'  is
          not a descendant of `D'.

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

        3 The only types involved in the Parent clauses are
          classes or formal generic parameters of `D'.