oo programing question

[shoko2004@hotmail.com (shoko)]

i have the following code:

package a is
  --car class --
  type a is tagged private; 
  type a_ptr is access all car'class;
  
  --car methods --
   private
     type a is tagged 
     record
         name:String(1..256);
     end record;
  
   
end a;
-------------------------------------

with a;

package b is
   type b is new a.a with private;   <--  constraint error 
   type b_Ptr is access all b'Class;

   function init_b(maximum:positive) return b_Ptr;
  
  private
     type b is new  a.awith
     record
         maximum_:positive;
     end record;
end b;
-------------------------------------------
in the main procedure:

with a;use a;
with b;use b;

procedure main is
   new_b:b_ptr;
     
begin
    new_b := init_b(90);
 end main;

i get constraint error on b.ads
i dont understand why

please help