Differences with/without .all

Hello people,

Gnat is giving me an error depending on if I use the .all abbreviation or
not.
I had the (unresearched) idea that there should be no differences besides
the
aesthetic one[*], so I'm not sure if gnat's wrong or I'm (guess what ;) )

One way or the other, I'd like to know the explanation. The particular
case
that is giving me problems in GPL 2007 is:

protected type Safe;
type Safe_Access is access all Safe;
function S return Safe_Access;

S.Some_Procedure;
--  This fails with
--  Prefix of protected procedure or entry call must be variable

S.all.Some_Procedure;
--  This compiles and runs OK.

Your judgment?

Alex.

[*] Exception possibly being constructions of this kind:
S.all'Access
which may be not equivalent to the original access type ("S'Type")?