@J2ktIncompatible
    @Override
    Object writeReplace() {
      return new KeysSerializedForm(ImmutableMultimap.this);
    }

    @GwtIncompatible
    @J2ktIncompatible
    private void readObject(ObjectInputStream stream) throws InvalidObjectException {
      throw new InvalidObjectException("Use KeysSerializedForm");
    }
  }

  @GwtIncompatible
  @J2ktIncompatible