code: public static <T> DecomposableMatchBuilder1<List<T>, T> headNil(MatchesAny<T> head) {
      List<Matcher<Object>> matchers = Lists.of(ArgumentMatchers.any());
      return new DecomposableMatchBuilder1<List<T>, T>(
          matchers, 0, new ListConsHeadFieldExtractor<>());
    }
docs: Matches a list with exactly one element.
  
  <p>If matched, the {@code head} value is extracted.