docs: static Function<Optional<Descriptor>, String> fieldScopeFunction(
        final String fmt, final FieldScope fieldScope) {
      return new Function<Optional<Descriptor>, String>() {
        @Override
        public String apply(Optional<Descriptor> optDescriptor) {
          return String.format(fmt, fieldScope.usingCorrespondenceString(optDescriptor));
        }
      };
    }