docs: @Nullable
      public static <T> TtlCallable<T> get(@Nullable Callable<T> callable, boolean releaseTtlValueReferenceAfterCall, boolean idempotent) {
          if (null == callable) return null;
  
          if (callable instanceof TtlEnhanced) {
              // avoid redundant decoration, and ensure idempotency
              if (idempotent) return (TtlCallable<T>) callable;
              else throw new IllegalStateException("Already TtlCallable!");
          }
          return new TtlCallable<T>(callable, releaseTtlValueReferenceAfterCall);
      }