return new KeysSerializedForm(ImmutableMultimap.this);
    }

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

  @GwtIncompatible
  @J2ktIncompatible
  private static final class KeysSerializedForm implements Serializable {

      return new KeysSerializedForm(ImmutableMultimap.this);
    }

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

  @GwtIncompatible
  @J2ktIncompatible
  private static final class KeysSerializedForm implements Serializable {