code: public static Set<? extends Element> annotatedElementsIn(
        RoundEnvironment roundEnv, Class<? extends Annotation> a) {
      return Sets.filter(roundEnv.getElementsAnnotatedWith(a),
          element -> element.getAnnotation(a) != null);
    }
docs: Sanitizes the result of {@link RoundEnvironment#getElementsAnnotatedWith}, which otherwise
  can contain elements annotated with annotations of ERROR type.
  
  <p>The canonical example is forgetting to import &#64;Nullable.