Serialized type for all ImmutableBiMap instances. It captures the logical
   * contents and they are reconstructed using public factory methods. This
   * ensures that the implementation types remain as implementation details.
Serialized type for all ImmutableBiMap instances. It captures the logical
   * contents and they are reconstructed using public factory methods. This
   * ensures that the implementation types remain as implementation details. | ImmutableBiMap.java