docs: @Nullable
      public static TtlRunnable get(@Nullable Runnable runnable, boolean releaseTtlValueReferenceAfterRun, boolean idempotent) {
          if (null == runnable) return null;
  
          if (runnable instanceof TtlEnhanced) {
              // avoid redundant decoration, and ensure idempotency
              if (idempotent) return (TtlRunnable) runnable;
              else throw new IllegalStateException("Already TtlRunnable!");
          }
          return new TtlRunnable(runnable, releaseTtlValueReferenceAfterRun);
      }