code: public static <U, V> Converter<U, V> wrapSingletonList(Converter<List<U>, V> converter) {
      Converter<U, List<U>> listConverter = new SingletonListConverter<U>().inverse();
      return listConverter.compose(converter);
    }
docs: Gets a converter which wraps a converter taking a {@code List} with only
  one element and eliminates the need for the {@code List}.
  
  @param converter
  @return