Re: Is T an ancestor of T?

["Björn Persson" <spam-away@nowhere.nil>]

Georg Bauhaus wrote:

> So we know there is exactly one ultimate ancestor for each type T.
> If T is at the root of a derivation hierarchy of height 0,
> is there a named type that is the ultimate ancestor of T?
> Is it T? By 10.d I would say yes.

Yes.

> By the last sentence of (10),
> the ultimate ancestor (T) "is not a descendant of any other type".
> So if T is the ultimate ancestor, it is not a descendant of T.
> Which it is by the first sentence of (10).
> I must be missing something. Specific types?

Maybe it's the word "other" you're missing? If it read "not a descendant 
of any type", then there would be a contradiction as you say, but it 
reads "not a descendant of any *other* type". "Other" must mean "other 
than itself".

> Then if D is new T with ..., is T the ultimate ancestor of D?
> (Again I would agree that it is.)

Yes.

> If T is the ultimate ancestor of T (and T is the ultimate
> ancestor of D if D had been defined), can GNAT be right when
> (T with n => 0) is accepted as an extension aggregate of specific
> type T? I still don't think so.

Me neither.

By the way, did you notice that class-wide types aren't ancestors or 
descendants of themselves?

-- 
Björn Persson                              PGP key A88682FD
                    omb jor ers @sv ge.
                    r o.b n.p son eri nu