derived types in package specs

[blakemor@software.ORG (Alex Blakemore)]

  Does anyone have an idea of the rationale behind RM 3.4(15) ?

  "If a derived type or private type is declared immediately within
   the visible part of a package, then, within this visible part,
   this type must not be used as the parent type of a derived
   type definition." ... see also 7.4.1.(4)

  This disallows things like
    type this is new integer;
    type that is new this;

  and what really hurt was
    type this is private;
    that that is new this;

  The draft copy of the Ada design rationale doesn't seem to discuss this.
  If this were legal, I think I could come up with a very nice use for
  derived types (something that has been rare but not unheard of in my experience)
  So there must have been a reason right ?

                                  Alex Blakemore
                                  Software Productivity Consortium
                                  blakemore@software.org