Generic body

Hi,

I'm trying to create a generic procedure with some instance in the same 
package.

package MyPack is

   generic
     type Toto is limited private;
   procedure MyProc;

   procedure MyProcInt is new MyProc(Toto => Integer);
   procedure MyProcStr is new MyProc(Toto => String);

end MyPack;

package body MyPack is

   procedure MyProc is
   begin
     -- Some stuff
   end MyProc;

end MyPack;

I don't how to do this because:
1) The compiler refuse the instance of MyProc because it doesn't have 
the MyProc body
2) If I put the body of MyProc in the package spec, the compiler refuses 
it because it's not the right place.

Thanks by advance for any help

Sebastien