@Override
            public ImmutableSet<Element> process(
                ImmutableSetMultimap<String, Element> elementsByAnnotation) {
              for (Element element : elementsByAnnotation.values()) {
                generateClass(processingEnv.getFiler(), element.getSimpleName() + "XYZ");
              }
              return ImmutableSet.of();
            }
When annotating a type {@code Foo}, generates a class called {@code FooXYZ}.
common/src/test/java/com/google/auto/common/BasicAnnotationProcessorTest.java