Since the bimap is immutable, ImmutableBiMap doesn't require special logic
   * for keeping the bimap and its inverse in sync during serialization, the way
   * AbstractBiMap does.
Since the bimap is immutable, ImmutableBiMap doesn't require special logic
   * for keeping the bimap and its inverse in sync during serialization, the way
   * AbstractBiMap does. | ImmutableBiMap.java