use constant U => ....;
    sub init   {@A = (0) x U}
    sub insert {$A [shift] = 1}
    sub delete {$A [shift] = 0}
    sub query  {$A [shift]}
    sub clear  {@A = (0) x U}

##</code><code>##

       A              B

    +------+       +------+
U-1 |  ?   |       |  ?   |
    +------+       +------+
    .      .       .      .
    .      .       .      .
    .      .       .      .
    .      .       .      .
    .      .       .      .
    +------+       +------+
 4  |  0   |       |  3   |
    +------+       +------+
 3  |  4   |       |  1   |  <---- C (== 3)
    +------+       +------+  
 2  |  1   |       |  1   |
    +------+       +------+
 1  |  2   |       |  2   |
    +------+       +------+  
 0  |  2   |       |  4   |
    +------+       +------+


##</code><code>##

    int query (int i)   {0 <= A [i] && A [i] < C && B [A [i]] == i}
    void insert (int i) {if (!query (i)) {B [C] = i; A [i] = C; C ++}}
    void delete (int i) {if (query (i)) {
                             B [A [ i]] = B [C - 1];
                             A [B [ C - 1]] = A [i];
                             C --}}