code: public static <T, ET extends T, A1, A2> DecomposableMatchBuilder2<List<T>, A1, A2> headNil(
        DecomposableMatchBuilder2<ET, A1, A2> head) {
      List<Matcher<Object>> matchers = Lists.of(ArgumentMatchers.any());
      return new DecomposableMatchBuilder1<List<T>, ET>(
          matchers, 0, new ListConsHeadFieldExtractor<>()).decomposeFirst(head);
    }
docs: Matches a list with exactly one element.
  
  <p>If matched, the {@code head} value is decomposed to 2.