docs: @Nonnull
      public static <T> List<Callable<T>> unwraps(@Nullable Collection<? extends Callable<T>> tasks) {
          if (null == tasks) return Collections.emptyList();
  
          List<Callable<T>> copy = new ArrayList<Callable<T>>();
          for (Callable<T> task : tasks) {
              if (!(task instanceof TtlCallable)) copy.add(task);
              else copy.add(((TtlCallable<T>) task).getCallable());
          }
          return copy;
      }