code: public static <A, B> Callback<A> transform(Callback<? super B> callback, Function<? super A, ? extends B> transformer)
      {
          return new TransformedCallback<A, B>(callback, transformer);
      }
docs: Create a new Callback which transforms its items according to a {@link Function}
  and then invokes the original callback.
  @param callback the callback that processes items of type B
  @param transformer the transformer that converts the item from type A to type B
  @param <A> the type of item that will be provided as input
  @param <B> the type of item processed by the provided callback