code: public static <T, K, V> Collector<T, ?, ImmutableBiMap<K, V>> toImmutableBiMap(
        Function<? super T, ? extends K> keyFunction,
        Function<? super T, ? extends V> valueFunction) {
      return CollectCollectors.toImmutableBiMap(keyFunction, valueFunction);
    }
docs: Returns a {@link Collector} that accumulates elements into an {@code ImmutableBiMap} whose keys
  and values are the result of applying the provided mapping functions to the input elements.
  Entries appear in the result {@code ImmutableBiMap} in encounter order.
  
  <p>If the mapped keys or values contain duplicates (according to {@link Object#equals(Object)},
  an {@code IllegalArgumentException} is thrown when the collection operation is performed. (This
  differs from the {@code Collector} returned by {@link Collectors#toMap(Function, Function)},
  which throws an {@code IllegalStateException}.)
  
  @since 21.0