@Override
    public ImmutableSet<? extends Element> process(
        SetMultimap<Class<? extends Annotation>, Element> elementsByAnnotation) {
      this.elementsByAnnotation = elementsByAnnotation;
      return ImmutableSet.of();
    }
Report an error for any class annotated.
common/src/test/java/com/google/auto/common/BasicAnnotationProcessorTest.java