Aspect location in expression function.

[Blady <p.p11@orange.fr>]

Hello,

I'm puzzled when I want to changed a function body with aspects to an 
expression function, for instance:

function Length (S : Some_Tagged_Tyoe) return Natural
   with Pre => S.Valid
   is
   begin
   return S.Length;
   end;

have to be changed in:

function Length (S : Some_Tagged_Tyoe) return Natural
   is (S.Length)
   with Pre => S.Valid;

The location of the aspect has moved to the end.

I'd like simply replace the begin block by the expression, as:

function Length (S : Some_Tagged_Tyoe) return Natural
   with Pre => S.Valid
   is (S.Length);

What could be any reasons not to permit it?

Thanks, Pascal.