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);
  }
}