VUPR
        A Precursor is valid if and only if it satisfies the 
        following five conditions:
 
            1. It appears in the Routine_body or the Rescue_clause
            of the declaration of exactly one routine `r' of C.

            2. If the Parent_qualification part is present, its
            Class_name is the name of a parent P of C.

            3. The declaration of `r' in C is the redefinition of a 
            routine that has exactly one version inherited by C as
            effective in the set of Parent types of C, restricted
            if case 2 applies to those having P as their Class_name.

