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

       A              B

...
 0  |  2   |       |  4   |
    +------+       +------+

    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 ++}}
...
                             B [A [ i]] = B [C - 1];
                             A [B [ C - 1]] = A [i];
                             C --}}