int a, b; /* put this at the top! */


    a = clock[startIndex];
    b = clock[finishIndex];

    clock[startIndex] = clock[startIndex] ^ clock[finishIndex];
    clock[finishIndex] = clock[startIndex] ^ clock[finishIndex];
    clock[startIndex] = clock[startIndex] ^ clock[finishIndex];

    assert(b == clock[startIndex]);
    assert(a == clock[finishIndex]);