A builder for creating immutable bimap instances, especially {@code public
   * static final} bimaps ("constant bimaps"). Example: <pre>   {@code
   *
   *   static final ImmutableBiMap<String, Integer> WORD_TO_INT =
   *       new ImmutableBiMap.Builder<String, Integer>()
   *           .put("one", 1)
   *           .put("two", 2)
   *           .put("three", 3)
   *           .build();}</pre>
   *
   * <p>For <i>small</i> immutable bimaps, the {@code ImmutableBiMap.of()} methods
   * are even more convenient.
   *
   * <p>Builder instances can be reused - it is safe to call {@link #build}
   * multiple times to build multiple bimaps in series. Each bimap is a superset
   * of the bimaps created before it.
A builder for creating immutable bimap instances, especially {@code public
   * static final} bimaps ("constant bimaps"). Example: <pre>   {@code
   *
   *   static final ImmutableBiMap<String, Integer> WORD_TO_INT =
   *       new ImmutableBiMap.Builder<String, Integer>()
   *           .put("one", 1)
   *           .put("two", 2)
   *           .put("three", 3)
   *           .build();}</pre>
   *
   * <p>For <i>small</i> immutable bimaps, the {@code ImmutableBiMap.of()} methods
   * are even more convenient.
   *
   * <p>Builder instances can be reused - it is safe to call {@link #build}
   * multiple times to build multiple bimaps in series. Each bimap is a superset
   * of the bimaps created before it. | ImmutableBiMap.java