svoid allowDeferredComputation(Runnable r) {
  try {
    callF(r);
  } catch (RuntimeException e) {
    Throwable e2 = innerException(e);
    if (!e2 instanceof DeferredComputationException) throw rethrow(e);
  }
}

sclass DeferredComputation {
  new Flag done;
  O value;
  Throwable error;
  Runnable continuation, onError;

  void setContinuation(Runnable r) {
    synchronized(done) {
      if (!done.isUp()) ret with continuation = r;
    }
    if (error == null)
      callF(r);
    else
      if (onError != null)
        callF(onError);
      else
        printStackTrace(error);
  }

  void setValue(O value) {
    synchronized(done) {
      if (done.isUp()) fail("setValue/setError called twice");
      this.value = value;
      done.raise();
    }
    callF(continuation);
  }

  void setError(Throwable error) {
    synchronized(done) {
      if (done.isUp()) fail("setValue/setError called twice");
      this.error = error;
      done.raise();
    }
    if (onError != null)
      callF(onError);
    else
      printStackTrace(error);
  }

  O getValue() {
    assertTrue(done.isUp());
    ret value;
  }
}

// f1: func -> A, f2 : func(A) -> B, returns: B
static O chainDeferrableComputations(O f1, fO f2) {
  O result;
  try {
    result = callF(f1);
  } catch (RuntimeException e) {
    Throwable e2 = innerException(e);
    if (!e2 instanceof DeferredComputationException) throw rethrow(e);
    final DeferredComputation f = e2/DeferredComputationException.future;
    
    final new DeferredComputation future2;
    f.setContinuation(r {
      try {
        future2.setValue(callF(f2, f.getValue()));
      } catch e {
        future2.setError(e);
      }
    });
    throw DeferredComputationException(future2);
  }
  ret callF(f2, result);
}

static <A> A waitThenCompute(int delay, final F0<A> f) {
  final new DeferredComputation dc;
  doLater(delay, r {
    try {
      dc.setValue(callF(f));
    } catch e {
      dc.setError(e);
    }
  });
  throw DeferredComputationException(dc);
}

sclass DeferredComputationException extends QuickException {
  DeferredComputation future;

  *(DeferredComputation *future) {}
}