*     then this is not logically a {@code Converter} at all, and should just implement {@link
   *     Function}.
   */
  @ForOverride
  protected abstract A doBackward(B b);

  // API (consumer-side) methods

  /**
   * Returns a representation of {@code a} as an instance of type {@code B}.
   *
   * @return the converted value; is null <i>if and only if</i> {@code a} is null
   */
  @CheckForNull

   *     then this is not logically a {@code Converter} at all, and should just implement {@link
   *     Function}.
   */
  @ForOverride
  protected abstract A doBackward(B b);

  // API (consumer-side) methods

  /**
   * Returns a representation of {@code a} as an instance of type {@code B}.
   *
   * @return the converted value; is null <i>if and only if</i> {@code a} is null
   */
  @CheckForNull