code: 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));
        }
      };
    }
docs: Returns a function which formats the given string by getting the usingCorrespondenceString from
  the given FieldScope with the argument descriptor.
  
  @param fmt Format string that must contain exactly one '%s' and no other format parameters.