* and values are the result of applying the provided mapping functions to the input elements.
   *
   * <p>If the mapped keys contain duplicates (according to {@link Object#equals(Object)}), the
   * values are merged using the specified merging function. If the merging function returns {@code
   * null}, then the collector removes the value that has been computed for the key thus far (though
   * future occurrences of the key would reinsert it).

   * @deprecated Unsupported operation.
   */
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  @CheckForNull
  public final V merge(
      K key, V value, BiFunction<? super V, ? super V, ? extends @Nullable V> function) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.