@Override
  public final ImmutableSet<String> getSupportedAnnotationTypes() {
    checkState(steps != null);
    return steps.stream().flatMap(step -> step.annotations().stream()).collect(toImmutableSet());
  }
Returns the set of supported annotation types as collected from registered {@linkplain Step
processing steps}.
common/src/main/java/com/google/auto/common/BasicAnnotationProcessor.java