Impossible to extend native types?

I am surprised: it doesn't seem possible to extend BlitzMax type like String

Type StringExt Extends String
End Type

This does not compile!
What if I want to extend the String object to add some extra-functionnality to it?? That could be VERY useful!!

Is it really not possible? Whyyyy??

maybe its declared Final

The code for the BlitzMAX string type is written in C, not BlitzMAX. Perhaps that is the problem.

Interestingly, quite a few other languages make the string type final.