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 --}}