sclass SteppableOnce is Steppable {
  Runnable action;
  
  public bool step() {
    Runnable r = action;
    if (r != null) {
      action = null;
      r.run();
    }
    false;
  }
}

static Steppable steppableOnce(Runnable action) {
  ret new SteppableOnce(action);
}