如何在knitr RMD文件中设置不同的全局选项

我想在RMD文件中排除许多代码块,并想用 eval = FALSE 之前和之后 eval = TRUE 之后但这是行不通的。

下面是一个玩具示例:

---
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = FALSE, eval = FALSE)
```
## will not be executed because global option `eval = FALSE`
```{r}
x+y
```

```{r setup II, include=FALSE}
knitr::opts_chunk$set(eval = TRUE, echo = TRUE)
```
## Should give `2` in final document, because eval = TRUE
## NOT working as expected
```{r}
1+1
```

## This is working as expected
```{r, eval = TRUE}
1+1
```

结果如下:

这是一个bug还是一个特性?在我看来,这是违反直觉的。。。