premature usage of incomplete private type

[robert-j@gmx.de (Robert Jäschke)]

Hello!

I'm writing a package for Rational-Arithmetics and there I've got a problem.
I want to declare a constant of type Rational before the type itself is
proper defined and I don't know how I can go around this problem:


package My_Math.Rational is

      type Rational is private;

      Zero : constant Rational := (Z => 0, N => 1);

      --  some declarations ...

private

      type Rational is record
            Z : Integer;  --  Z�hler
            N : Integer;  --  Nenner
      end record;

end My_Math.Rational;


If I try to compile this specification, GNAT gives me this error:

gnatgcc -c -gnatg my_math-rational.ads
my_math-rational.ads:10:07: premature usage of incomplete private type
"Rational" defined at line 8
my_math-rational.ads:10:35: expected private type "Rational" defined at line
8
my_math-rational.ads:10:35: found a composite type
gnatmake: "my_math-rational.ads" compilation error


How can I solve this problem without making Rational non-private? Is there
any way, to say the compiler, that this type will be defined later?

Thanks for your help, 

Robert