sclass WaitForStableValue<A> {
  long period;
  A value;
  long valueSince;
  
  *() {}
  *(long *period) {}
  *(double seconds) { period = toMS(seconds); }
  
  synchronized A set(A a) {
    if (neq(a, value)) {
      value = a;
      valueSince = sysNow();
    }
    ret get();
  }
  
  synchronized A get() {
    ret valueSince != 0 && sysNow() >= valueSince+period ? value : null;
  }
  
  // clears only if result != null
  synchronized A setAndClear(A _a) {
    A a = set(_a);
    if (a != null) clear();
    ret a;
  }
  
  public bool has() { ret get() != null; }
  
  void clear() { set(null); }
}