code: public static <T> DecomposableMatchBuilder0<List<T>> headNil(MatchesExact<T> head) {
      List<Matcher<Object>> matchers = Lists.of(ArgumentMatchers.eq(head.t));
      return new DecomposableMatchBuilder0<List<T>>(matchers, new ListConsHeadFieldExtractor<>());
    }
docs: Matches a list with exactly one element.