static ImmutableSet<ExecutableElement> toPrettyStringMethods(Context context) {
    return context.abstractMethods().stream()
        .filter(method -> toPrettyStringAnnotation(method).isPresent())
        .collect(toImmutableSet());
  }
Returns the {@link com.google.auto.value.extension.toprettystring.ToPrettyString} annotated
methods for an {@code @AutoValue} type.
value/src/main/java/com/google/auto/value/extension/toprettystring/processor/ToPrettyStringMethods.java