generic
   type T is private;
package Stack is
   type Stack is private;

   -- Push a value into the stack.
   procedure Push (S : in out Stack; Val : T);

   -- Pop a value from the stack.
   function Pop (S : in out Stack; Val : out T) return Boolean;

   -- Return true if the stack is empty, false otherwise.
   function Empty (S : Stack) return Boolean;
private
   type Node;
   type Node_Access is access Node;

   type Node is record
      Val    : T;
      Bottom : Node_Access;
   end record;

   type Stack is record
      Top : Node_Access;
   end record;
end Stack;