code: @NotNull
      @Contract("_ -> new")
      public static <T> Exceptional<T> of(@NotNull Throwable throwable) {
          Objects.requireNonNull(throwable);
          return new Exceptional<T>(null, throwable);
      }
docs: Returns an {@code Exceptional} with throwable already set.
  
  @param <T> the type of value
  @param throwable  throwable instance
  @return an {@code Exceptional}