sclass PingSource {
  // returns true if it slept
  settable volatile IF0<Bool> action;

  // returns true if it slept
  final bool get() {
    var a = action;
    ret a != null && a!;
  }
  
  void cancel {
    action = new Cancelled;
  }
  
  sclass Cancelled is IF0<Bool> {
    public Bool get() { throw new PingSourceCancelledException(this); }
  }
}