code: public List<Excerpt> getNestedClasses() {
      if (nestedClasses instanceof ImmutableList) {
        nestedClasses = new ArrayList<>(nestedClasses);
      }
      return Collections.unmodifiableList(nestedClasses);
    }
docs: Returns an unmodifiable view of the list that will be returned by {@link
  Datatype#getNestedClasses()}. Changes to this builder will be reflected in the view.