sclass PastValueKeeper<A> {

  long forgetAfter;

  Lock lock = lock();

  L<Pair<A, Long>> values = synchroList();

  *() {}

  *(double seconds) { forgetAfter = toMS(seconds); }

  void add(A a) {

    lock lock;

    if (nempty(values) && eq(last(values).a, a)) ret;

    clean();

    values.add(pair(a, machineTime()));

  }

  A valueAt_orFirst(long timeBack) {

    lock lock;

    clean();

    long time = machineTime()-timeBack;

    int i = 0;

    while (i < l(values) && values.get(i).b < time)

      ++i;

    ret pairA(get(values, max(i-1, 0)));

  }

  void clean {

    lock lock;

    long time = machineTime()-forgetAfter;

    int i = 0;

    while (i < l(values) && values.get(i).b < time)

      ++i;

    removeSubList(values, 0, i);

  }

}