Modular types inside records

[niewiap@widzew.net (Pawe� Niewiadomski)]

I have an assignment to implement a normal, unidirectional queue as a 
static array of n_max elements.  I wanted the array to be "cyclic", namely 
I wanted to use two variables: head and tail of a modular type within the 
queue record so that there would be no constraint error unless the array 
were really full (checked by an appropriate function).  If the tail index 
were on the last element of the array, its value after the incrementation 
would be 0, and by that time the 0 element of the array would be probably 
freed.

generic
    type item_type is private;
package queues is
    type queue(n_max: positive) is tagged private;
    function empty (q: queue) return boolean;
    function full (q: queue) return boolean;
    procedure add (q: queue; item: in item_type);
    procedure remove (q: queue; item: out item_type);
private
    type arr is array (natural range <>) of item_type;
    type queue (n_max: positive) is tagged record
        values: arr (0..n_max);
        tail: natural:=0; --I want these two be of modular type with 
        head: natural:=1; --a modulus equal to n_max+1
    end record;
end queues;

If you have any idea on how to solve this problem (no dynamic memory 
allocation, please) I would be grateful for sharing it with me
Pawel