I have **Googled** and found these resources:

* <https://www.firegiant.com/wix/tutorial/com-expression-syntax-miscellanea/expression-syntax/>
* <http://alekdavis.blogspot.com/2013/05/wix-woes-what-is-your-installer-doing.html>

But I am still not sure if what I want to add is correct syntax or not. Is this valid?

```
<Custom Action="CAInstall" After="CAInstall.SetCustomActionData">NOT Installed or REINSTALL="ALL" or NOT (REMOVE="ALL")</Custom>

```

The part I am thinking to add is the last condition:

```
or NOT (REMOVE="ALL")

```