super(inputFuture, function);
    }

    @Override
    ListenableFuture<? extends O> doTransform(
        AsyncFunction<? super I, ? extends O> function, @ParametricNullness I input)
        throws Exception {
      ListenableFuture<? extends O> output = function.apply(input);
      checkNotNull(
          output,

  /**
   * Returns a {@code Future} whose result is taken from this {@code Future} or, if this {@code
   * Future} fails with the given {@code exceptionType}, from the result provided by the {@code
   * fallback}. {@link AsyncFunction#apply} is not invoked until the primary input has failed, so if
   * the primary input succeeds, it is never invoked. If, during the invocation of {@code fallback},