code: public static <Head, Tail extends HList> HCons<Head, Tail> cons(Head head, Tail tail) {
          return new HCons<>(head, tail);
      }
docs: Static factory method for creating an HList from the given head and tail.
  
  @param head   the head element
  @param tail   the tail HList
  @param <Head> the head type
  @param <Tail> the tail type
  @return the newly created HList