We have a value group of boolean value, like this: PrimaryPage.UISelection(ABC) = true

What method shall I call to retrieve the true/false value? I came up with following statement, but it doesn't seem to work. They say "The method findPage(String) is undefined for the type ClipboardPage"

tools.getPrimaryPage().findPage("UISelection").getProperty("ABC")

Can I use findPage() to retrieve valueList/ValueGroup, does them considered a spacial kind of Clipboard Page? If I don't use it what shall I use?

Blahblahblah: It's so intimidating that Pega defined so many Domain Specified languages, e.g. the Expressions syntax, When Rule methods, JSP, Engine API, Public API. It can be helpful in long run, but as a junior developer, right now I have no idea what's the linkage in between. Also, Is there any guideline documents catering this topic?