Iterable<String> result = FluentIterable.from(input).transform(new StringValueOfFunction());

    assertEquals(asList("1", "2", "null", "3"), Lists.newArrayList(result));
  }

  private static final class RepeatedStringValueOfFunction
      implements Function<Integer, List<String>> {
    @Override
    public List<String> apply(Integer from) {
      String value = String.valueOf(from);
      return ImmutableList.of(value, value);
    }